UI Pages Refactoring

[Confectrician]

    Now that UI add-ons docs are available some documentation could be further deduplicated:

E.g. replace:

• https://www.openhab.org/docs/ui/habot/#habot
  with
  https://www.openhab.org/addons/ui/habot/

• https://www.openhab.org/docs/configuration/habpanel.html
  with
  https://www.openhab.org/addons/ui/habpanel/doc/habpanel.html (needs some fixing to get the images loaded)

Originally posted by @wborn in #1935 (comment)

[Confectrician]

We should remove the specific ui pages here and merge/sync them with the addon readmes before doing so.
Also we should then temporary add 301 redirects from docs to addons via website repository, to inform google about those changes.

We should be able to leave the sidebar elements for a while and redirect directly to the adons pages.

[wborn]

Yes that would be nice! There's also some link replacement logic in prepare-docs.rb that may need an update for this.