Rework stackforge.html for infra manual changes

Rework the page so that all content that is now
in the infra-manual is removed and the page just explains
what StackForge is.

Replace links to stackforge with a link to the project creators
page.

Change-Id: I79902876d05f8fc540e18bd3f596ea1f0cdf0e30
Rename voting-testers group to <project>-ci

We ended up going with <project>-ci as the name for these third party
testing CI group names. Update the doc to reflect this change.

Change-Id: If96603f35313dcd6d38cb0e5f1736daa2a4170be
Merge "Rework stackforge.html for infra manual changes"