docs: add Diátaxis documentation types to README standards

[joshschmelzle]

Problem

The README standards cover README structure only. There is no rule for pages beyond the README, so repos that outgrow it end up with flat docs/ folders that mix user guides, API facts, design notes, and working plans (wlanpi-core, wlanpi-mcp, wlanpi-profiler, and others).

Change

Adds a Documentation Types section to style/README_STANDARDS.md based on Diátaxis:

• Each page serves one reader need: tutorial, how-to, reference, or explanation. Content for another need goes on its own linked page.
• The README stays the entry point. Once docs outgrow it, use docs/{tutorials,how-to,reference,explanation}/, creating only the folders that have pages.
• Plans, PRDs, specs, and handover notes stay outside the four type folders.
• Troubleshooting is a how-to page.
• Pages are fixed when touched for another reason, not rewritten in bulk.

REPOS.md records that no repo uses the layout yet and lists the six with ad-hoc docs/ folders.

No existing repo has to change now.

Validation

• shellcheck install.sh: pass
• actionlint: pass