ci, docs: fix documentation text and skip docs-only CI

[moluopro]

• Fix several documentation typos, wording issues, broken Markdown formatting, and links.
• Run the docs workflow only for documentation-related inputs:
  • docs/**
  • mkdocs.yml
  • .github/workflows/docs.yml
• Skip non-doc CI workflows when a change only touches documentation files:
  • docs/**
  • **/*.md

[moluopro]

I hate those unnecessary CI checks. They take way too long.