Fix the unresolved Sphinx references in the mmap documentation by:

• documenting the access, page-size, and PROT_* constants with data directives;
• qualifying the mmap.seek reference;
• correcting the combined PROT_READ | PROT_WRITE references; and
• removing mmap.rst from .nitignore.

The documented constants were verified against Modules/mmapmodule.c.

Validation:

• Targeted Sphinx build completed with no warnings.
• Full documentation HTML build passed in nitpicky mode with warnings treated as errors.
• The documentation warning regression check passed.
• The targeted mmap.rst nitpicky Sphinx build completed with no warnings.
• The full CPython documentation warning comparison found no new regressions.

• Issue: Fix Sphinx reference warnings in various Doc/library/ files #151950