Feature/add heat on demand docs

[KJonline]

Proposed change

Add heat on demand documentation.

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 hive heat on demand core#48591
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue:

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.

[probot-home-assistant]

It seems that this PR is targeted against an incorrect branch since it has a parent PR on one of our codebases. Documentation that needs to be updated for an upcoming release should target the next branch. Please change the target branch of this PR to next and rebase if needed.
_{^{(message by DocsTargetBranch)}}

[frenck]

✅ Approved. Can be merged as soon as the parent PR gets merged.

[KJonline]

Hi @frenck

Please can this be merged as the parent PR is now merged in?

On a different note the 2021.4.0 release adds UI setup for the Hive integration but it seems to be missing from the beta release notes.

Not sure if the xiaomi is just a placeholder and a full list will be added to the official release notes, apologies if it is.