Add catch_dotenv hook to prevent committing .env files

[rwbot]

This pull request introduces a new pre-commit hook to prevent accidental commits of .env files containing sensitive information, along with supporting documentation, configuration, and testing resources. The main addition is the catch-dotenv hook, which blocks .env files from being committed, optionally generates a sanitized .env.example, and ensures .env is listed in .gitignore.

New pre-commit hook for dotenv protection:

• Added the catch-dotenv hook implementation in pre_commit_hooks/catch_dotenv.py, which blocks commits of .env files, can generate a .env.example with only variable names, and ensures .env is present in .gitignore.
• Registered the new hook in .pre-commit-hooks.yaml with a description and configuration options.
• Documented usage, options, and benefits of catch-dotenv in README.md.

Integration and testing resources:

• Added catch-dotenv to the setup.cfg entry points to enable command-line usage and integration with pre-commit.
• Included a sample .env file with dummy secrets in testing/resources/test.env for testing.

[asottile]

hi in the future please discuss features before spending time on them

you don't really need any code for this. this can be done trivially with a language: fail hook