content/documentation: put docs in the tree

See merge request !125

Gets rid of ReadTheDocs even for 5.x, and also acknowledges the
existence of 6.x.

Inspired by how Knot DNS does it, however we are going to keep `latest`
as a proxy to GitLab Pages and possibly, when 6.1 is out, generate that
automatically from the `master` branch of Knot Resolver, similarly to
the way ReadTheDocs did it.

release 5.7.2

See merge request !124

Definitely a sane default to not return non-zero on error... 🤦

release 5.7.1

See merge request !121

This whole thing will be dropped soon via MR !120

release 5.7.0

See merge request !119

knot/knot-resolver#798

release 5.5.3

See merge request !116

release 5.5.2

See merge request !115

release 5.5.0

See merge request !112

make serve: use Python 3

See merge request !111

Simplify Donations setup

See merge request !110

Force the Donations app to use English

See merge request !109

Use Python 3.9 in CI

See merge request !108