Don't deploy docs when working on a fork (#13278)

# Objective

- Some developers enable Github Actions for their fork and commit
directly to main. This triggers the `docs.yml` action, which attempts to
deploy the documentation even if Github Pages is not enabled. (It also
creates a `CNAME` file specific to Bevy and should not be used in forks,
even for testing.)
- For an example, see [this
run](https://github.com/tychedelia/bevy/actions/runs/8978912060/job/24660082729).

## Solution

- Only attempt to deploy docs when running from the main Bevy
repository.
- This does not affect us checking `cargo doc` on pull requests, since
that it done in `ci.yml`.

## Testing

It's difficult to test this, but you'd probably:

1. Fork Bevy
2. Cherry pick this PR's commits onto the main branch of your fork.
3. Push another commit to the main branch, triggering Github Actions.
4. Check the Github Actions job summary to ensure that the
`build-and-deploy` job is skipped.

.github/workflows/docs.yml

@@ -27,6 +27,9 @@ concurrency:
 jobs:
   build-and-deploy:
     runs-on: ubuntu-latest
+    # Only run this job when on the main Bevy repository. Without this, it would also run on forks
+    # where developers work on the main branch but have not enabled Github Pages.
+    if: ${{ github.repository == 'bevyengine/bevy' }}
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}