🏗️🔧 add support for JEKYLL_GITHUB_TOKEN env var

[DerekNonGeneric]

The jekyll-github-metadata gem is automatically enabled for GitHub Pages builds (including builds using this gem locally). It pulls in information from the GitHub API, but first checks the environment for authentication, which is required for some API calls to return the data we want. It's a warning, not an error, so you should be good to ignore them locally. 👍
— github/pages-gem#399 (comment)

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"GitHub Metadata: No GitHub API authentication could be found. Some fields may be missing or have incorrect data."

To create a personal access token: (public_repo scope is enough for public repository): https://docs.github.com/en/github/authenticating-to-github/keeping-your-account-and-data-secure/creating-a-personal-access-token

Refs: github/pages-gem#399

[DerekNonGeneric]

I am going to land this momentarily, but if anyone gets stuck, here are the docs:

• https://github.com/jekyll/github-metadata/blob/main/docs/authentication.md#authentication

There is a followup PR to be made as it seems this should be documented (perhaps in the file being added by #611).