feat(docs): deploy main branch preview alongside PR previews (#707)

## Summary

- Extend the docs-preview workflow to also trigger on pushes to `main`,
deploying a continuously-updated preview to `_preview/pr-main/` on
`gh-pages`
- Rename the umbrella directory from `pr-preview` to `_preview`
- Scope concurrency per-ref (`docs-preview-${{ github.ref }}`) with
`cancel-in-progress: true`

## Details

Production deploys via Craft remain unchanged. This adds a `main` branch
preview so merged docs changes are immediately viewable at
`cli.sentry.dev/_preview/pr-main/` without waiting for a release.

The workflow reuses `rossjrw/pr-preview-action` for both cases with
conditional inputs:
- **Push to main**: `action: deploy`, `pr-number: main`, `comment:
false`
- **PR events**: `action: auto` (unchanged behavior)

The astro config's landing page regex is updated to match the new
`_preview` path and the `main` preview.

Old `pr-preview/` directory on `gh-pages` is left as-is (harmless, will
be cleaned up by future Craft releases).

Author: BYK

.github/workflows/docs-preview.yml

@@ -1,6 +1,15 @@
 name: Docs Preview
 
 on:
+  push:
+    branches: [main]
+    paths:
+      - 'docs/**'
+      - 'src/**'
+      - 'script/generate-command-docs.ts'
+      - 'script/generate-skill.ts'
+      - 'install'
+      - '.github/workflows/docs-preview.yml'
   pull_request:
     paths:
       - 'docs/**'
@@ -14,6 +23,10 @@ permissions:
   contents: write
   pull-requests: write
 
+concurrency:
+  group: docs-preview-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   preview:
     runs-on: ubuntu-latest
@@ -41,8 +54,9 @@ jobs:
       - name: Build Docs for Preview
         working-directory: docs
         env:
-          # Override base path for PR preview (no /cli prefix since preview domain is different)
-          DOCS_BASE_PATH: /pr-preview/pr-${{ github.event.pull_request.number }}
+          DOCS_BASE_PATH: ${{ github.event_name == 'push'
+            && '/_preview/pr-main'
+            || format('/_preview/pr-{0}', github.event.pull_request.number) }}
           PUBLIC_SENTRY_ENVIRONMENT: staging
           SENTRY_RELEASE: ${{ steps.version.outputs.version }}
           PUBLIC_SENTRY_RELEASE: ${{ steps.version.outputs.version }}
@@ -102,6 +116,8 @@ jobs:
         with:
           source-dir: docs/dist/
           preview-branch: gh-pages
-          umbrella-dir: pr-preview
+          umbrella-dir: _preview
           pages-base-url: cli.sentry.dev
-          action: auto
+          action: ${{ github.event_name == 'push' && 'deploy' || 'auto' }}
+          pr-number: ${{ github.event_name == 'push' && 'main' || github.event.pull_request.number }}
+          comment: ${{ github.event_name != 'push' }}

docs/astro.config.mjs

@@ -82,7 +82,7 @@ export default defineConfig({
                 const path = window.location.pathname;
                 // Works with both / (prod) and /pr-preview/pr-XX (preview)
                 return path === '/' || 
-                       /^\\/pr-preview\\/pr-\\d+\\/?$/.test(path);
+                       /^\\/_preview\\/pr-(\\d+|main)\\/?$/.test(path);
               }
               
               function checkAtBottom() {