docs: extract to a separate repository, switch to SDoc documentation by default

[stanislaw]

When the documentation for Read The Docs is regenerated, we get very large diffs like this one: https://github.com/strictdoc-project/strictdoc/pull/892. It does not make sense to store the auto-generated content along with the codebase.

The documentation itself can stay within the main repository but the generated artefacts should be pushed to another one.

[stanislaw]

https://github.com/peter-evans/repository-dispatch might be useful here.

[stanislaw]

It turns out that StrictDoc's own documentation can be deployed to Read the Docs along with the Sphinx HTML/PDF docs.

There is no need for a separate repo for the time being. Closing this in favor of https://github.com/strictdoc-project/strictdoc/pull/2023.