Docusaurus [sidebar
generation](https://docusaurus.io/docs/next/sidebar/autogenerated)
expects category index pages to have one of three file path conventions:
- `section/index.mdx`
- `section/README.mdx`
- `section/section.mdx`
This change standardizes category index paths on the third convention so
Docusaurus sidebar generation succeeds. We can then add checks to the
current docs site to prevent additional menu pages from violating this
convention.
This change also adds redirects to the new category index pages, and
updates internal links to pages that were moved.
Note that this change does not move all relevant menu pages. We still
need to reorganize the `reference/terraform-provider` section. Since
this section is automatically generated, we need another approach to
restructuring it.
