Document Detekt static analysis for Android

[loganrosen]

Proposed change

Document how Detekt complements KTLint in the Android repository and how contributors can run its Gradle tasks. The guidance also explains the baseline behavior that allows the new analysis to block regressions without requiring unrelated existing findings to be fixed first.

This accompanies home-assistant/android#7209 and intentionally avoids naming a Detekt version so the documentation remains accurate as the tooling is updated.

Type of change

• Document existing features within Home Assistant
• Document new or changing features for which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Editing or restructuring documentation guidelines
• Changes to the backend of this documentation
• Remove stale or deprecated documentation

Checklist

• I have read and followed the documentation guidelines.
• I have verified that my changes render correctly in the documentation.

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: Add Detekt static analysis android#7209