[codex] Document supported MapCSS runtime features

[Komzpa]

Summary

• add a maintained in-repo manual for the MapCSS feature surface that Kothic JS actually consumes after conversion
• document supported selectors, geometry types, layering, paint properties, text/icon/shield options, eval helpers, localization, and runtime limitations
• document latitude-aware metric(...) / zmetric(...) behavior from [codex] Scale MapCSS metric units by tile latitude #87
• link the manual from the README and fix a small typo in the locale paragraph

Fixes #22.

Validation

• npm install
• npm test
• git diff --check

Notes

• The metric/zmetric paragraph assumes [codex] Scale MapCSS metric units by tile latitude #87 lands before or together with this docs PR.