revise and proofread project documentation

[wackbyte]

• README.md:
  • Edit for grammar and clarity.
• CONTRIBUTING.md:
  • Edit as above.
  • Remove unnecessary horizontal rules.
  • Move the shared "commit guideline" section up to reduce duplicated content.
  • Format repository/project structure blocks as lists rather than code. I think this is more accessible and easier to read.
  • Use GitHub Markdown's "alert" feature.
• docs/:
  • Edit as above.
  • Replace HTML <hr> with Markdown --- and remove unnecessary ones.
  • Remove some extraneous <br>.
• etc.

This PR is not exhaustive. I may do more work in a follow-up PR.