Add new Mintlify documentation site (docs_new/) (#23001)

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docs_new/docs_migration_plan.md

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+# SGLang Documentation Migration Plan
+
+## Background
+
+Migrate the new Mintlify-based documentation (currently in the standalone `sgl-docs` repo) into the sglang main repo under `docs_new/`, and point `staging.docs.sglang.io` to it.
+
+### Current State
+
+| Item | Location | Stack | Domain |
+|------|----------|-------|--------|
+| Old docs | `sglang/docs/` | Sphinx + GitHub Pages | `docs.sglang.io` |
+| New docs + cookbook | `sgl-project/sgl-docs` repo | Mintlify | `lmsysorg.mintlify.app` (temp preview) |
+
+- Cookbook is already inside `sgl-docs/cookbook/`, no separate repo needed.
+- Old docs CI (`execute-notebook.yml`, `lint.yml`) only watches `docs/**`, will not be triggered by `docs_new/**`.
+
+---
+
+## Phase 1: Git Subtree Merge (Local Experiment)
+
+> Goal: Merge `sgl-docs` into `sglang` repo's `docs_new/` directory, preserving full commit history and authorship.
+
+```bash
+# 1. Create a new branch (sglang remote is NOT affected)
+cd /path/to/sglang
+git checkout -b docs-new-migration
+
+# 2. Add sgl-docs as a remote (sgl-docs repo is NOT affected, read-only fetch)
+git remote add sgl-docs git@github.com:sgl-project/sgl-docs.git
+git fetch sgl-docs
+
+# 3. Subtree merge — all sgl-docs content goes into docs_new/, full history preserved
+git subtree add --prefix=docs_new sgl-docs main
+
+# 4. Keep the remote for ongoing sync during migration period
+#    (remove only after sgl-docs is officially archived)
+```
+
+### Safety Guarantees
+
+- `sgl-docs` original repo: **unaffected** (fetch only, no push)
+- `sglang` remote: **unaffected** (local branch, no push until ready)
+- Rollback: `git checkout main && git branch -D docs-new-migration`
+
+### Side Effect: Contributors
+
+`git subtree add` (without `--squash`) imports all original commits. Authors from `sgl-docs` will appear in `sglang`'s git history and GitHub Contributors list. This is intentional — it gives proper credit.
+
+---
+
+## Phase 2: Configure Mintlify for `docs_new/` (on branch)
+
+> Goal: Make Mintlify read from `sglang` repo's `docs_new/` subdirectory instead of the standalone `sgl-docs` repo. **No need to merge to main first** — Mintlify can point to a specific branch for validation.
+
+1. Log in to [Mintlify Dashboard](https://dashboard.mintlify.com)
+2. Change the project's **GitHub repository** from `sgl-project/sgl-docs` to `sgl-project/sglang`
+3. Set **Branch** to `docs-new-migration` (temporarily, for validation)
+4. Set **Documentation directory** to `docs_new` (Mintlify supports monorepo subdirectory)
+5. `docs.json` (Mintlify config) will be at `docs_new/docs.json` after the subtree merge — paths inside it (e.g., `cookbook/llm/Qwen/Qwen3`) are relative to `docs_new/`, so no changes needed
+6. Verify the preview build succeeds on Mintlify
+
+---
+
+## Phase 3: DNS & Custom Domain for `staging.docs.sglang.io`
+
+> Goal: Make `staging.docs.sglang.io` serve the new Mintlify docs.
+
+1. **DNS**: Add a CNAME record for `staging.docs.sglang.io` pointing to Mintlify's endpoint (typically `cname.mintlify.dev`)
+2. **Mintlify Dashboard**: Settings > Custom Domain > add `staging.docs.sglang.io`
+3. Mintlify handles SSL certificate automatically
+4. Verify `staging.docs.sglang.io` loads correctly
+
+---
+
+## Phase 4: Ongoing Sync During Migration Period
+
+> During the transition, `sgl-docs` may still receive updates. Sync them into `docs_new/` as needed.
+
+```bash
+# Pull latest changes from sgl-docs into docs_new/
+git subtree pull --prefix=docs_new sgl-docs main
+```
+
+Once `sgl-docs` is frozen, this step is no longer needed.
+
+---
+
+## Phase 5: CI/CD (Optional, Post-Migration)
+
+Current `docs/**` CI workflows will **NOT** trigger for `docs_new/**` changes. This is fine initially since Mintlify has its own GitHub integration for auto-deployment on push to main.
+
+Optional additions later:
+- Link checking (lychee) for `docs_new/**/*.mdx`
+- Mintlify broken-link or build validation on PR
+
+---
+
+## Phase 6: Final Cutover
+
+> Goal: Promote staging to production.
+
+| Stage | `docs.sglang.io` | `staging.docs.sglang.io` |
+|-------|-------------------|--------------------------|
+| After Phase 3 | Sphinx (old docs) | Mintlify (new docs) |
+| After cutover | Mintlify (new docs) | Keep or remove |
+
+Cutover steps:
+1. Confirm `staging.docs.sglang.io` is stable and content-complete
+2. Update DNS: point `docs.sglang.io` CNAME from GitHub Pages to Mintlify (`cname.mintlify.dev`)
+3. Update Mintlify Dashboard custom domain to `docs.sglang.io`
+4. Remove or archive old resources:
+   - Delete `sglang/docs/` (old Sphinx docs)
+   - Delete `.github/workflows/release-docs.yml` and `.github/workflows/execute-notebook.yml`
+   - Archive `sgl-project/sgl-docs` repo on GitHub
+   - Remove the `sgl-docs` git remote: `git remote remove sgl-docs`
+   - Optionally archive `sgl-project/sgl-project.github.io` repo
+
+---
+
+## Execution Order
+
+> Mintlify supports pointing to a specific branch, so we can validate on `docs-new-migration` **before** merging to main.
+
+| Step | Action | Who | Dependency |
+|------|--------|-----|------------|
+| 1 | Phase 1: subtree merge on local branch | Dev | — |
+| 2 | Push branch to `sgl-project/sglang` | Dev | Step 1 |
+| 3 | Phase 2: configure Mintlify Dashboard to read from `sgl-project/sglang` branch `docs-new-migration` `docs_new/` | Admin (Mintlify access) | Step 2 |
+| 4 | Phase 3: DNS CNAME + Mintlify custom domain for `staging.docs.sglang.io` | Admin (DNS access) | Step 3 |
+| 5 | Verify staging site | Team | Step 4 |
+| 6 | Merge PR to main, switch Mintlify branch back to `main` | Dev + Admin | Step 5 confirmed OK |
+| 7 | Phase 4: sync any remaining sgl-docs updates | Dev | As needed |
+| 8 | Phase 6: final cutover when ready | Admin | Step 6 done |