Add documentation of pvvx_mithermometer display platform

[puuu]

Description:

Documentation for pvvx_mithermometer display platform.

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#3333

Checklist:

• [x] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
• [x] Link added in /index.rst when creating new documents for new components or cookbook.

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[github-actions[bot]]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[puuu]

For the bot: the parent esphome/esphome#3333 pull request is still alive.

[github-actions[bot]]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[puuu]

For the bot: the parent https://github.com/esphome/esphome/pull/3333 pull request is still alive.