fix: remove collections markdownlint exclusion

[MauroDruwel]

Summary

• remove markdownlint exclusion for collections/*.collection.md
• fix markdown violations in collection markdown and regenerated plugin READMEs
• update collection markdown generation to emit lint-compliant section headings and replace em dashes in generated descriptions
• clarify frontmatter validator exclusion rationale for generated collection markdown files

Validation

• npx markdownlint-cli2 "collections/*.collection.md"
• npm run plugin:generate
• npm run plugin:postprocess
• npm run lint:md

[MauroDruwel]

Closing in favour of #1372 which covers the same scope on a better-named branch. No need for two competing PRs.