chore: three MANIFEST.in lines match nothing #1794

Describe the bug
Three MANIFEST.in lines have no effect. Each was checked by building sdists with and without the line; the listings are identical (1248 entries).

• :3 global-include *.rst: no tracked .rst outside docs/ ships since chore: drop stale build leftovers and move the examples' READMEs to Markdown (#1773) #1780; the 37 shipped .rst files come from the recursive-include of docs/source/changelog.
• :88 include pcapkit/_extern: include matches files, not a directory; every build warns no files found matching. This holds even with a fake _extern tree present, as in the conda build.
• :89 exclude pcapkit/protocols/*/NotImplemented: matches nothing; every build warns no previously-included files found, and all 37 NotImplemented paths ship either way.

Expected behavior
Each line either does what it says or is removed, and the build warnings go away. docs/source/contributing/conventions/documentation.rst:270-278, which records that the placeholders ship, should still agree.

Additional context
Found by the #1103 second pass (repo slice) and confirmed by a Sonnet review.