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/
```

docs/source/conf.py

@@ -10,7 +10,8 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
-# import os
+import os
+
 # import sys
 # sys.path.insert(0, os.path.abspath('.'))
 
@@ -138,9 +139,15 @@ suppress_warnings = ["epub.unknown_project_files"]
 
 sphinx_tabs_valid_builders = ["epub", "linkcheck"]
 
+# -- Options for linkcheck -------------------------------------------------
+
 # Excluded links for linkcheck
 # These should be periodically checked by hand to ensure that they are still functional
-linkcheck_ignore = ["https://www.raspberrypi.com/software/"]
+linkcheck_ignore = [R"https://www.raspberrypi.com/software/", R"http://10\..+"]
+
+token = os.environ.get("GITHUB_TOKEN", None)
+if token:
+    linkcheck_auth = [(R"https://github.com/.+", token)]
 
 # MyST configuration (https://myst-parser.readthedocs.io/en/latest/configuration.html)
 myst_enable_extensions = ["colon_fence"]