Update the manual page with new options

This pull request updates the rustc manual page to represent current state of rustc option handling better. Moved the apparently deprecated options (#19900) to their own section and added all the new codegen options.

A bit unrelatedly, I also updated description of `-O` and `-g` flags to point to the new codegen options rather than old, deprecated ones.

Fixes #20111.
I’m not sure where to put `--pretty` then. Maybe just move it back to the list of regular options? It might also be sensible to omit it until it is stabilised (if at all, ever).

As for the rest, does leaving just `--debuginfo` and `--opt-level`, since they sound like the most commonly used ones, sound good?