docs: document additional_dependencies for pre-commit plugin hooks

[akashchamp]

Description

Documents that a third-party commitizen plugin (e.g. cz-conventional-gitmoji) must be listed under additional_dependencies in .pre-commit-config.yaml when used with the commitizen pre-commit/prek hook. prek/pre-commit installs each hook in its own isolated environment, so a plugin that isn't part of commitizen itself is not importable unless declared, and the hook fails with The committer has not been found in the system.

Adds a "Using a third-party commitizen plugin" tip (with a working additional_dependencies example) right after the existing .pre-commit-config.yaml example in the pre-commit setup tutorial, since that's the doc page people follow to configure the hook. No code changes.

Closes #1141

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude (Anthropic, Claude Code) following the guidelines. I reviewed the change, reproduced the bug, and verified the documented fix myself (see "Steps to Test" below).

Code Changes

• Add test cases to all the changes you introduce — N/A, documentation-only change (no code behavior changed)
• Run uv run poe all locally to ensure this change passes linter check and tests — N/A code paths untouched; ran uv run mkdocs build --strict and uv run prek run --files docs/tutorials/auto_check.md instead (see below)
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes — this PR is the documentation update

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

The pre-commit setup tutorial should tell readers that a third-party commitizen plugin needs to be added to additional_dependencies, so they don't hit The committer has not been found in the system the way the issue reporter did.

Steps to Test This Pull Request

1. Reproduced the exact bug from the issue: created a scratch repo with pyproject.toml containing [tool.commitizen]\nname = "cz_gitmoji", and a .pre-commit-config.yaml pointing the commitizen hook at this repo without additional_dependencies. Ran git commit with stages: [commit-msg] installed via prek install --hook-type commit-msg:

   commitizen check.........................................................Failed
   - hook id: commitizen
   - exit code: 1
   
     The committer has not been found in the system.
   
     Try running 'pip install cz_gitmoji'
   

   This matches the issue's reported failure exactly.
2. Added additional_dependencies: ["cz-conventional-gitmoji"] to the same hook config (the fix this PR documents) and re-ran the commit:

   commitizen check.........................................................Passed
   [master 83bfd29] feat: add gitmoji feature
   

   Commit succeeded (exit code 0).
3. Built the docs to confirm the new admonition renders correctly and all links resolve: uv run mkdocs build --strict → exit code 0, no warnings.
4. Ran the repo's pre-commit hooks against the changed file: uv run prek run --files docs/tutorials/auto_check.md → all applicable hooks (trailing-whitespace, end-of-file-fixer, blacken-docs, codespell, etc.) passed.

Additional Context

Issue: #1141. The maintainer (@Lee-W) confirmed in the issue thread that this is a documentation gap ("this is indeed something we can improve in the documentation"), and another user (@atiasn) confirmed in a follow-up comment that adding additional_dependencies: ["cz-conventional-gitmoji"] resolved the problem for them — this PR documents exactly that fix.