description of rustdoc test suites is scattered

[lolbinarycat]

There are at least 3 pages that describe the structure of the tests/rustdoc* folders.

1. https://rustc-dev-guide.rust-lang.org/tests/compiletest.html#rustdoc-test-suites
2. https://rustc-dev-guide.rust-lang.org/rustdoc.html#tests
3. https://rustc-dev-guide.rust-lang.org/rustdoc-internals.html#dotting-is-and-crossing-ts

1 links to 2, but otherwise there is no way to navigate between them. Either they should all link to each other, or they should be condensed into a single page (perhaps with some 1 line sections left behind for people who follow links to the old locations)