Search: stop creating SphinxDomain objects (#10451)
We are indexing sphinx domains as sections now, we no longer need to create sphinx domains or index them. We should see some small improvement in build time, and maybe in search?
This change will require a re-index, since old projects that haven't been re-build still have sphinx domain objects indexed separately from sections (this step can be done after or before the deploy).
This also fixes a small bug, we are still indexing sphinx domains titles, without their content
ref https://github.com/readthedocs/readthedocs.org/issues/10272
