Updated granularity of docs summary

[UebelAndre]

I've done a pass on both starlark code comments and the mdbook page to try and improve search-ability within the book. The general direction of this change is to fix outdated starlark docs and then add more sub-pages to surface things in SUMMARY.md so they pop out at a glance.

[georgesfarah]

LGTM

see: #4191

crate_universe_workspace.md was never listed in docs/src/SUMMARY.md, so mdbook never generated that page.

[georgesfarah]

cargo_bootstrap_repository and cargo_dep_env are orphaned — both get public wrapper .bzl files created, but neither gets a stardoc target, neither is added to the mdbook srcs, and neither appears in SUMMARY.md.

They were both documented in the old monolithic cargo.md. This is a doc regression — they silently lose their pages.