docs: show dotenv usage in pre-commit Python hooks

[HaloJun]

Add a runnable local-hook example showing how python-dotenv[cli] in additional_dependencies makes dotenv run -- available inside pre-commit's isolated Python environment. Explain the required .env file, default override behavior, and --no-override.

Closes #484

Validation: the exact README YAML passes pre-commit validate-config and runs successfully in a real isolated hook environment. The same child command fails without dotenv; the current checkout CLI loads the variables, and --no-override preserves an existing value. MkDocs strict build and rendered README checks pass, with tracked docs symlinks materialized in a separate Windows build copy. git diff --check passes.

AI assistance: prepared with Codex; human review is pending. This changes only README documentation; no library behavior changes or full library test run.