docs: docusaurus setup (#3541)

# What does this PR do?

- Docusaurus server setup
- Deprecates Sphinx build pipeline
- Deprecates remaining references to Readthedocs
- MDX compile errors and broken links to be addressed in follow-up PRs

<!-- Provide a short summary of what this PR does and why. Link to relevant issues if applicable. -->

<!-- If resolving an issue, uncomment and update the line below -->

<!-- Closes #[issue-number] -->

## Test Plan

```
npm install
npm gen-api-docs all
npm run build
```

<!-- Describe the tests you ran to verify your changes with result summaries. *Provide clear instructions so the plan can be easily re-executed.* -->

.github/workflows/README.md

@@ -21,4 +21,3 @@ Llama Stack uses GitHub Actions for Continuous Integration (CI). Below is a tabl
 | Test External API and Providers | [test-external.yml](test-external.yml) | Test the External API and Provider mechanisms |
 | UI Tests | [ui-unit-tests.yml](ui-unit-tests.yml) | Run the UI test suite |
 | Unit Tests | [unit-tests.yml](unit-tests.yml) | Run the unit test suite |
-| Update ReadTheDocs | [update-readthedocs.yml](update-readthedocs.yml) | Update the Llama Stack ReadTheDocs site |

.github/workflows/update-readthedocs.yml

@@ -1,70 +0,0 @@
-name: Update ReadTheDocs
-
-run-name: Update the Llama Stack ReadTheDocs site
-
-on:
-  workflow_dispatch:
-    inputs:
-      branch:
-        description: 'RTD version to update'
-        required: false
-        default: 'latest'
-  push:
-    branches:
-      - main
-    paths:
-      - 'docs/**'
-      - 'pyproject.toml'
-      - '.github/workflows/update-readthedocs.yml'
-    tags:
-      - '*'
-  pull_request:
-    branches:
-      - main
-    paths:
-      - 'docs/**'
-      - 'pyproject.toml'
-      - '.github/workflows/update-readthedocs.yml'
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.ref == 'refs/heads/main' && github.run_id || github.ref }}
-  cancel-in-progress: true
-
-jobs:
-  update-readthedocs:
-    runs-on: ubuntu-latest
-    env:
-      TOKEN: ${{ secrets.READTHEDOCS_TOKEN }}
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-
-      - name: Install dependencies
-        uses: ./.github/actions/setup-runner
-
-      - name: Build HTML
-        run: |
-          cd docs
-          uv run make html
-
-      - name: Trigger ReadTheDocs build
-        if: github.event_name != 'pull_request'
-        run: |
-          if [ -z "$TOKEN" ]; then
-            echo "READTHEDOCS_TOKEN is not set"
-            exit 1
-          fi
-
-          response=$(curl -X POST \
-            -H "Content-Type: application/json" \
-            -d "{
-              \"token\": \"$TOKEN\",
-              \"version\": \"$GITHUB_REF_NAME\"
-            }" \
-            https://readthedocs.org/api/v2/webhook/llama-stack/289768/)
-
-          echo "Response: $response"
-          if [ $(echo $response | jq -r '.build_triggered') != 'true' ]; then
-            echo "Failed to trigger ReadTheDocs build"
-            exit 1
-          fi