Addons: make default root CSS selector a shared option (#11767)
Add a "global" shared CSS root selector that is used all across our
features: link previews, docdiff, search, embed API, etc.
Also, put the "documentation tool name" as a top level config as well.
With that information we can define better defaults for CSS selectors
and other stuffs in the future if needed.
Example: if documentation tool is set as Sphinx by the user, we will be
able to use `[role=main] a.internal` for the Link Preview CSS selector.
Related https://github.com/readthedocs/addons/pull/433