Addons: API response for tooltips (#11746)
Initial `addons.tooltips` object with basic data for testing purposes.
We will also want to add here:
- [x] ext-theme addons configurations for tooltips
- [x] model fields for tooltips
- [x] disable it by default for now
- [x] tests for the new API response
Required by https://github.com/readthedocs/addons/pull/422/
Closes https://github.com/readthedocs/addons/issues/239
