Migrate imap_email_content to config entry setup

[jbouwh]

Proposed change

Migrate imap_email_content integration for config entry setup. Existing config from configuration.yaml is migrated automatically to config entries, so there is no need any more to document the yaml config.

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: Migrate imap_email_content integration to config entry core#89707
• 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.

[jbouwh]

Superseded by home-assistant/core#90429 and #26792