[3.12] Improve cross-references in `runpy` docs (GH-107673) (#107698)

Improve cross-references in `runpy` docs (GH-107673)

- Add links to `__main__` and `sys.path` where appropriate
- Ensure each paragraph never has more than one link to the same thing, to avoid visual clutter from too many links
(cherry picked from commit 4e242d1ffb2d165443fe2680f7d1fef9fecbcfc0)

Co-authored-by: Kamil Turek <kamil.turek@hotmail.com>