Allow Publish Docs to be run manually (#1289)

## Summary

Adds a manual trigger (`workflow_dispatch`) to **Publish Docs**, so docs.vapi.ai can be republished from the Actions tab or the CLI without merging a PR.

## Why

The nightly `update-openapi` job auto-merges its spec PR with the built-in `GITHUB_TOKEN`. GitHub doesn't start other workflows from events made with that token, so **Publish Docs never runs for spec updates**. New API fields therefore don't reach the live API reference until someone merges an unrelated docs PR.

Example: voice simulation latency expectations shipped on Oct 3. The spec updates that added the fields merged in #1284 (Oct 3) and #1286 (Oct 6), but the last publish ran on Oct 2, so the live Create and Update Scenario pages still don't show `latencyExpectations`.

## Change

Two lines in `.github/workflows/publish-docs.yml`:

- add `workflow_dispatch:` under `on:`;
- widen the job's `if:` guard to accept `workflow_dispatch` as well as `push`. Without this, a manual run would show as skipped. The guard also now checks `github.ref == 'refs/heads/main'` exactly instead of `contains(...)`, so a manual run can't publish another branch.

The publish steps and secrets are unchanged.

## Using it

Once merged, anyone with write access can run **Actions → Publish Docs → Run workflow** on `main`, or:

```bash
gh workflow run publish-docs.yml --repo VapiAI/docs --ref main
```

Merging this PR also publishes everything pending on `main`, including the latest API spec.

## Not included

This makes publishing possible on demand but doesn't make it automatic after the nightly spec update. Two options for the docs owners:

- merge the nightly PR with a GitHub App or bot account token instead of `GITHUB_TOKEN`, so the merge triggers Publish Docs;
- or add a `schedule:` to this workflow shortly after the nightly update.

## Testing

`actionlint` v1.7.7 passes with no findings, and the YAML parses with both triggers. A manual trigger can only be exercised once it's on `main`, so the first real test is a manual run after merge.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: scott-lowe-vapi

.github/workflows/publish-docs.yml

@@ -4,11 +4,12 @@ on:
   push:
     branches:
       - main
+  workflow_dispatch:
 
 jobs:
   run:
     runs-on: ubuntu-latest
-    if: ${{ github.event_name == 'push' && contains(github.ref, 'refs/heads/main') && github.run_number > 1 }}
+    if: ${{ (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == 'refs/heads/main' && github.run_number > 1 }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4