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.
That's probably a good point, let's just omit this whole section for now. Since the `--debuginfo` and `--opt-level` options have been renamed (and will be removed soon), I think we can remove them as well.