Add a to-do integration documentation

[allenporter]

Proposed change

Add a to-do integration documentation. Also adds a To-do List category for integrations.

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 architecture proposal: New entity component: `todo` architecture#962
• Link to core pull request: Add todo component core#100019
• Link to frontend pull request: Add todo component that supports multiple To-do lists. frontend#17883
• Link to brands pull request: Add icon for todo component brands#4682
• Link to developer documentation pull request: Add todo component developer documentation developers.home-assistant#1929

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]

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