docs: Update GetStartedGuide with extra repos info

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Add Plugins and OutputPlugins documentation

To complement the xunit output plugin mode made by
Ruda.

CC: Ruda Moura <rmoura@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #37 from avocado-framework/documentation-updates

Documentation updates