Add documentation for custom auth implentations in application credentials

[allenporter]

Proposed change

Add documentation for the application credentials platform for providing a custom authentication implementation.

Type of change

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

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: Add application credentials platform for google calendar integration core#71808
• Related arch discussion: Revisit rules for OAuth usage in config flow (for Google) architecture#692