Capability matrix contains runners that aren't in the left nav or on master

Currently the runners on feature branches are in the capability matrix, but not linked to their summary pages or anything. Now we also have IBM Streams that could use a link so users can find what the column is about.

I think we can link to either Ongoing Projects or external pages for these.

We may want a left nav item "other runners" for those that we don't want to highlight as part of what the Beam project builds and ships, so we can actually put them in context a bit. This might tie in to BEAM-2888.

Imported from Jira [BEAM-3168](https://issues.apache.org/jira/browse/BEAM-3168). Original Jira may contain additional context.
Reported by: kenn.

Start by reviewing the capability matrix, its runner entries, the left navigation, Ongoing Projects, and the linked BEAM-2888 and BEAM-3168 context. Determine which feature-branch runners and IBM Streams need links or an “other runners” context, then verify that each matrix entry is discoverable and accurately represented.

Written by the indexing model from the issue text.