Add Caldav configuration flow documentation

[allenporter]

Proposed change

Add Caldav configuration flow documentation, and refresh much of the stale documentation to nudge users towards UI and triggers rather than yaml, offsets, and entity attributes. Some of this is copied from google calendar.

The motivation is to prepare for supporting VTODO objects from CalDAV to enable things like iCloud To-do lists.

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: Add config flow to CalDAV core#103215
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[MartinHjelmare]

Thanks!