This switches the docs tests from the `oss-zip` distribution to the
`zip` distribution so they have xpack installed and configured with the
default basic license. The goal is to be able to merge the
`x-pack/docs` directory into the `docs` directory, marking the x-pack
docs with some kind of marker. This is the first step in that process.
This also enables `-Dtests.distribution` support for the `docs`
directory so you can run the tests against the `oss-zip` distribution
with something like
```
./gradlew -p docs check -Dtests.distribution=oss-zip
```
We can set up Jenkins to run both.
Relates to #30665
73549281e8