docs: clean up setup and usage guide

[rtmalikian]

Closes #114

Summary

• Replace the stale README setup flow with a current GitHappens install, config, alias, and usage guide.
• Document the active CLI command surface, including issue/MR creation, review flow, AI review, summaries, incident reports, and last deployment lookup.
• Fix broken docs examples, typos, config paths, the malformed --auto_merge command, duplicate deployment docs, and template-key comments.
• Add a backup log for the edited docs files.

Verification

• git diff --check
• python3 -m py_compile gitHappens.py ai_code_review.py
• npx --yes markdownlint-cli --disable MD013 MD033 MD041 -- README.md