MapReduce runner page and capability matrix should clearly call out that it is on a feature branch

The page is here: https://beam.apache.org/documentation/runners/mapreduce/

It does not call out that it is on a feature branch, and it even demonstrates setting up a maven dependency for it, implying that it has releases, which it does not.

Similarly, the capability matrix should (until we have a larger revamp) make very clear which runners are not actually available to users in the form of released artifacts.

Imported from Jira [BEAM-3814](https://issues.apache.org/jira/browse/BEAM-3814). Original Jira may contain additional context.
Reported by: kenn.

Start with the MapReduce runner page at https://beam.apache.org/documentation/runners/mapreduce/ and identify the Maven dependency example and feature-branch status. Then locate the capability matrix, update both areas to distinguish unreleased artifacts from available releases, and verify that the documentation consistently reflects the runner's availability.

Written by the indexing model from the issue text.