Fix rate limiting in sphinx link checker (#1579)

Fixes rate-limit errors in the sphinx linkchecker on GitHub links caused
by a missing token.

```
-rate limited-   https://github.com/PhotonVision/photonvision/commits/master/ | sleeping...
-rate limited-   https://github.com/PhotonVision/photonvision/commits/master/ | sleeping...
(docs/advanced-installation/prerelease-software: line    9) broken    https://github.com/PhotonVision/photonvision/commits/master/ - 429 Client Error: Too Many Requests for url: https://github.com/PhotonVision/photonvision/commits/master/
```

.github/workflows/photonvision-docs.yml

@@ -12,6 +12,9 @@ on:
       - 'docs/**'
       - '.github/**'
 
+env:
+  GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
 jobs:
   build:
     runs-on: ubuntu-22.04