Update documentation

[rachel-fenichel]

This is part of the monorepo project.

• Update docs about how to contribute to plugins
• Update references to develop/master branch in core
• Add documentation about how version numbers for plugins work
• Add documentation about how to configure dependabot to always update blockly + plugins together
• Update READMEs where needed
  • Audit all links in plugin READMEs (can happen in parallel)

[jult]

And updating hints? Most users will have it installed and there's nothing on how to update to newer versions without having to re-install from a new git rebase/clone source.

[lizschwab]

For Add documentation about how version numbers for plugins work item, we can use the pattern we applied for docusaurus, which looks like this:

    groups:
      docusaurus:
        patterns:
          - '@docusaurus/*'

See documentation here:
https://docs.github.com/en/code-security/tutorials/secure-your-dependencies/optimizing-pr-creation-version-updates#grouping-related-dependencies-together