docs(standard): add RM3, a plain Links section to close the README

[lesnik512]

Adds RM3 to the standard. It says how a README ends: a ## Links list (docs site if there is one, PyPI, license), then the existing "Part of modern-python" section. It also says README headings should not carry emoji.

Today most repos end with one H2 heading per link, each with an emoji (## 📚 [Documentation](...)). GitHub's outline and the PyPI page list those as sections with no content. The footer was never written down, so repos have drifted:

• 6 repos have three link headings, 17 have two, and 4 have one.
• httpware puts them on one line with dots between.
• compose2pod and release-scope have none.

RM3 is a SHOULD rather than a dated MUST. It is cosmetic, and nothing in CI or tooling depends on it. The repo sweep to bring READMEs in line comes after the open humanizer PRs merge, so no README has two PRs open against it at once.

just test passes and mkdocs build --strict is clean.