Broken documentation links

[alexander-bauer]

Hello and thank you for your work on rules_jsonnet!

I've noticed that some of the documentation links are broken; it looks like you might be midway through reorganizing, but I wanted to bring attention to it since I'm having trouble finding the right docs.

• On the GitHub repository homepage, the About section links to https://bazelbuild.github.io/rules_jsonnet/, which leads to a 404 on GitHub pages.
• In the README, the Rule usage section links to https://github.com/bazel-contrib/rules_jsonnet/blob/master/docs/jsonnet.md, which no longer exists.
• In docs/index.html and in docs/jsonnet/jsonnet.html, there is a link to the Bazel central registry at https://registry.bazel.build/docs/rules_jsonnet, but this page says "No API Documentation Available."

[alexander-bauer]

For posterity and anyone else arriving here in the future, the latest available docs that I can find are here on the previous tag: https://github.com/bazel-contrib/rules_jsonnet/blob/0.7.2/docs/jsonnet.md.