Use the official Jekyll container to build the site

Use the official Jekyll container to build the site

Using a container makes it much easier to ensure consistency between
different developer environments, and even avoids devs needing to setup
an environment and deal with dependency issues.

- Update the README to describe how to test locally using the Jekyll
  container

- Update GitHub Actions to use the official Jekyll container

- Update GitHub Actions to run on pull requests, but with the publish
  step skipped, so that PRs can test that everything builds correctly
  before being merged

- Commit the Gemfile.lock file to freeze dependency versions

- Remove asciidoc descriptions for blockdiag, seqdiag, actdiag, and
  netdiag. The dependencies for these diagrams aren't easily added to
  the Jekyll container, we don't use any of them, and there is no
  obvious benefit to them over PlantUML or GraphViz

DAFFODIL-2595