Add ouman_eh_800 integration documentation

[Markus98]

Proposed change

Add documentation for the new Ouman EH-800 heating controller core integration.

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 logos 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 Ouman EH-800 heating controller integration core#169733
• Link to parent pull request in the Brands repository: Add ouman_eh_800 core integration brand assets brands#10247
• 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]

It seems that this PR is targeted against an incorrect branch. Documentation updates which apply to our current stable release should target the current branch. Please change the target branch of this PR to current and rebase if needed. If this is documentation for a new feature, please add a link to that PR in your description.

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[Copilot]

Pull request overview

Adds a new integration page for the upcoming Ouman EH-800 Home Assistant integration. This fits the _integrations docs set by introducing setup, requirements, supported functionality, and operational notes for a new local device integration.

Changes:

• Adds front matter and metadata for the new ouman_eh_800 integration page
• Documents prerequisites, setup parameters, supported sensors, polling behavior, and limitations
• Includes a security warning about the device’s unencrypted local HTTP interface