Prepare GitHub docs for shift to buf.build/docs

[jrinehart-buf]

Note: Do not merge. This cannot be merged until Protovalidate documentation is live on buf.build/docs.

Notes for reviewers:

• There are no code changes here: purely Markdown. It's probably easiest to view the updates pages by opening the branch in a second browser tab.

This PR:

• Updates the README for protovalidate-java to be in "hierarchy of developer needs" order intended to be consistent across all Protovalidate repositories:
  1. What's Protovalidate?
  2. How do I install it or find support for my language? (Supported Languages)
  3. How do I use it? (Documentation)
  4. How do I contribute? (Contribution)
  5. Related Sites
  6. Legal
• Updates bug report template to list Protovalidate before protoc-gen-validate

[pkwarren]

At some point we should add some more badges to this README including the latest version number and potentially a link to published Javadocs.