[Ignore] Individual page diagnostic proof of concept

[parlough]

This a really rough and hacky proof of concept exploring the possibility of each diagnostic being on its own page to see if it's a direction we want to go. If it is a direction we want to go, I will rewrite this setup and code to be much cleaner and performant, and also make the necessary changes in analyzer to make this process easier.

• The home diagnostic page is now located at tools/diagnostics.
  • This page no longer includes individual details, only the glossary and context definitions, as well as a definition list of the diagnostics themselves.
• Each diagnostic now has its own page in the format of /tools/diagnostics/diagnostic_name such as `tools/diagnostics/conflicting_constructor_and_static_member.
  • Currently the old diagnostic documentation is kept in its current state, but now that they are on their own page, we can make additions more easily
  • I added a new Jekyll/Liquid filter to insert the word-break opportunity element <wbr /> after underscores in the page h1 header, as the diagnostic names were too long and going off the page.
• The old diagnostic page still exists at tools/diagnostic-messages, but has the following modifications:
  • Has a JS script which reads the user's fragment and attempts to redirect them to the diagnostic's new page if it exists
  • If the user doesn't have JS enabled, they will see text which explains the documentation for that diagnostic has moved with a link to the new diagnostic documentation.
  • See /tools/diagnostic-messages#conflicting_constructor_and_static_member for an example of the redirect working
  • Disable JS temporarily and visit the previous link to see the existing page with redirect text
  • The page also has a yellow border indicating it's out of date
  • The page is hidden from the sitemap to prevent it from showing up on search
  • The page has a warning at the top linking to the new diagnostics article

[github-actions]

Visit the preview URL for this PR (updated for commit bbca002):

https://dart-dev--pr4560-feature-individual-p-sidzadhh.web.app

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