docs: REST Core-API: sensor widget, multi-button updates for remotes & activities

[zehnm]

New multi-button merge and replace endpoints for activities and remote-entities:

• Merge multiple button mappings: PATCH /activities/{entityId}/buttons, PATCH /remotes/{entityId}/buttons
• Replace button mappings: POST /activities/{entityId}/buttons, POST /remotes/{entityId}/buttons

This functionality is required for enhanced copy-paste actions in the web-configurator.

New sensor widget for the activity user interface. Configuration options:

• sensor_id: referenced sensor-entity. It is not required to include the sensor-entity in the activity.
• Show / hide sensor label. Disabled by default.
  • The widget text can be used as an alternative sensor label.
• Show / hide sensor unit. Enabled by default.

[zehnm]

I'll leave the PR open until the feature is implemented and tested in the web-configurator, in case we still have to adjust something.