rustdoc: Restructure all rustdoc-related chapters

[fmease]

1. Chapters about rustdoc's test suites shouldn't live in rustdoc-internals/ but in tests/ where all other test-related things are located.
2. The split between "rustdoc overview" and "rustdoc internals" is weird, "I don't know where to put things".
3. The "rustdoc overview" (in "How to build in run the compiler") contains a lot of things, a lot of things. It's not really an overview / a succinct but complete intro on how to get things going.
4. Then we have "internals" as well which IIRC duplicates some of the info given in the "overview" / mentions things that should be in an "overview" idk

Disclaimer: My memory is slightly hazy, these were my thoughts / mere feelings from ~1 week ago when I worked on #2298. I prolly didn't read everything as carefully as I could because I worked on other things but I remember being slightly frustrated / confused with the overall structure of rustdoc-related information etc.

I know this isn't very actionable right now, these comments come straight out of my mind, unprocessed. I'll revisit this at some point.