Broken documentation links

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."

Start by checking the repository About link, the README Rule usage link, docs/index.html, and docs/jsonnet/jsonnet.html against the current documentation layout. Verify which documentation and Bazel Central Registry destinations exist, then update or remove the broken references so every listed link reaches the intended content or a valid page.

Written by the indexing model from the issue text.