Rollup merge of #49028 - QuietMisdreavus:the-dark-forbidden-corners-of-rustdoc, r...

add an "unstable features" chapter to the rustdoc book

There are several rustdoc features that currently are undocumented, but also don't fit with the rest of the Rustdoc Book since they're also unstable. Some of these have corresponding feature gates and chapters in the Unstable Book, but many don't, and i wanted a place to talk about them officially.

Goal: talk about everything rustdoc can do that needs nightly

- [x] Feature gates (extensions to the doc attribute that can be caught by the compiler)
  - [x] doc(cfg)
  - [x] doc(masked)
  - [x] doc(spotlight)
  - [x] doc(include)
- [x] Command-line flags (features that require a CLI flag to use, where the flag itself is a `-Z` command or otherwise requires `-Z unstable-options` before rustdoc will accept it)
  - [x] markdown-before-content/markdown-after-content
  - [x] playground-url
  - [x] display-warnings
  - [x] crate-version
  - [x] linker
  - [x] sort-modules-by-appearance
  - [x] themes/theme-checker
  - [x] resource-suffix
  - [x] `-Z force-unstable-if-unmarked`
- [x] Nightly-gated functionality (features that are gated by requiring a nightly build without needing a CLI flag or a feature gate to unlock)
  - [x] intra-links
  - [x] error numbers for `compile_fail` doctests