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 elastic#30665

This will allow us to merge xpack docs into the regular docs directory!

Relates to elastic#30665

* elastic/master: (40 commits)
  [DOC] Extend SQL docs
  Immediately flush channel after writing to buffer (elastic#31301)
  [DOCS] Shortens ML API intros
  Use quotes in the call invocation (elastic#31249)
  move security ingest processors to a sub ingest directory (elastic#31306)
  Add 5.6.11 version constant.
  Fix version detection.
  SQL: Whitelist SQL utility class for better scripting (elastic#30681)
  [Docs] All Rollup docs experimental, agg limitations, clarify DeleteJob (elastic#31299)
  CCS: don't proxy requests for already connected node (elastic#31273)
  Mute ScriptedMetricAggregatorTests testSelfReferencingAggStateAfterMap
  [test] opensuse packaging turn up debug logging
  Add unreleased version 6.3.1
  Removes experimental tag from scripted_metric aggregation (elastic#31298)
  [Rollup] Metric config parser must use builder so validation runs (elastic#31159)
  [ML] Check licence when datafeeds use cross cluster search  (elastic#31247)
  Add notion of internal index settings (elastic#31286)
  Test: Remove broken yml test feature (elastic#31255)
  REST hl client: cluster health to default to cluster level (elastic#31268)
  [ML] Update test thresholds to account for changes to memory control (elastic#31289)
  ...