Docs: Improve contributor setup guidance

[skytin1004]

Summary

• Fix README setup wording and link the beginner installation guide.
• Add a new contributor quick start to CONTRIBUTING with setup, check, fork, and PR links.
• Rewrite the command-line installation guide into PyPI and repository-clone paths.
• Clean stale command-line guide wording around Vision configuration and old Markdown-only references.
• Add a docs regression test for the typo/stale-phrase cluster from Docs: Fix typos and improve documentation #45.

Validation

• python -m black --check tests/co_op_translator/test_docs_issue_45.py
• python -m ruff check tests/co_op_translator/test_docs_issue_45.py
• python -m pytest tests/co_op_translator/test_docs_issue_45.py -q
• mkdocs build --strict
• git diff --check

Closes #45