Docs: Consolidate getting started references

[skytin1004]

Summary

• Point README and getting_started pages at the MkDocs pages that now own full CLI, configuration, API, and examples reference material.
• Shorten duplicated command-line walkthrough content so it stays as an entry path instead of a second source of truth.
• Update GitHub Actions guides to remove stale Markdown-only and set-up-resources links, and make the default workflow text-only unless image translation is explicitly configured.
• Add a docs consistency regression test for canonical links, stale flags, and missing relative links.

Validation

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