Add link to docs for jekyll.yml

[nisbet-hubbard]

Currently, it’s not easy for users who are not already intimately familiar with Jekyll to figure out the different purposes that jekyll.yml and jekyll-gh-pages.yml each serve. Very few guides elsewhere on the internet actually point to jekyll.yml as a possible way to update Jekyll to version 4 and avail oneself of plugins not officially supported by GitHub.

The corresponding docs page at Jekyll, on the other hand, is buried under docs/continuous-integration (which isn’t a page itself) and can’t be discovered through the site’s nav bar.

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The following checklist is not applicable to this PR, which only updates the comments in the workflow.

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Tasks

For all workflows, the workflow:

• Should be contained in a .yml file with the language or platform as its filename, in lower, kebab-cased format (for example, docker-image.yml). Special characters should be removed or replaced with words as appropriate (for example, "dotnet" instead of ".NET").
• Should use sentence case for the names of workflows and steps (for example, "Run tests").
• Should be named only by the name of the language or platform (for example, "Go", not "Go CI" or "Go Build").
• Should include comments in the workflow for any parts that are not obvious or could use clarification.
• Should specify least privileged permissions for GITHUB_TOKEN so that the workflow runs successfully.

For CI workflows, the workflow:

• Should be preserved under the ci directory.
• Should include a matching ci/properties/*.properties.json file (for example, ci/properties/docker-publish.properties.json).
• Should run on push to branches: [ $default-branch ] and pull_request to branches: [ $default-branch ].
• Packaging workflows should run on release with types: [ created ].
• Publishing workflows should have a filename that is the name of the language or platform, in lower case, followed by "-publish" (for example, docker-publish.yml).

For Code Scanning workflows, the workflow:

• Should be preserved under the code-scanning directory.
• Should include a matching code-scanning/properties/*.properties.json file (for example, code-scanning/properties/codeql.properties.json), with properties set as follows:
  • name: Name of the Code Scanning integration.
  • creator: Name of the organization/user producing the Code Scanning integration.
  • description: Short description of the Code Scanning integration.
  • categories: Array of languages supported by the Code Scanning integration.
  • iconName: Name of the SVG logo representing the Code Scanning integration. This SVG logo must be present in the icons directory.
• Should run on push to branches: [ $default-branch, $protected-branches ] and pull_request to branches: [ $default-branch ]. We also recommend a schedule trigger of cron: $cron-weekly (for example, codeql.yml).

Some general notes:

• This workflow must only use actions that are produced by GitHub, in the actions organization, or
• This workflow must only use actions that are produced by the language or ecosystem that the workflow supports. These actions must be published to the GitHub Marketplace. We require that these actions be referenced using the full 40 character hash of the action's commit instead of a tag. Additionally, workflows must include the following comment at the top of the workflow file:

  # This workflow uses actions that are not certified by GitHub.
  # They are provided by a third-party and are governed by
  # separate terms of service, privacy policy, and support
  # documentation.
  

• Automation and CI workflows should not send data to any 3rd party service except for the purposes of installing dependencies.
• Automation and CI workflows cannot be dependent on a paid service or product.