docs: add default branch

[dgarcia360]

Adds default branch to the docs fix the "Edit this page" button.

[Bouncheck]

Branch change itself LGTM, but on several pages the path does not resolve to correct files for me.
For example for load balancing manual I get https://github.com/scylladb/java-driver/edit/scylla-3.x/docs/source/manual/load_balancing/index.md
while I assume I should end up on https://github.com/scylladb/java-driver/edit/scylla-3.x/manual/load_balancing/README.md
This seems like a separate issue though so I'm approving the branch change.

[Lorak-mmk]

What about the driver 4.x? Will the edit button just not work for it?

[Bouncheck]

I'm assuming that for 4.x we need similar PR for scylla-4.x branch

[Bouncheck]

Maybe that's wrong assumption though. How is final documentation page created? I'm not sure who maintains it. Is it a result of make multiversion or is it a combination of a build made on 3.x branch and a separate one on 4.x branch? Because in make multiversionpreview it does indeed result in pointing to 3.x branch for 4.x versions too.

Lorak-mmk raised an important question

[Bouncheck]

It seems that on 4.x branch the docs directory is a copy of 3.x one. Looks like one made for convenience sake and that we do not maintain different configuration for 4.x. So if we create multi version (including 4.x) documentation from 3.x branch we should not be defaulting to 3.x urls for all versions. Thanks for the catch @Lorak-mmk.

[dkropachev]

@dgarcia360 , could you please take a look at scylladb/sphinx-scylladb-theme#1156 before merging it.

[dgarcia360]

Replaced by #367