Add Analytics reference to Capella

The Analytics documentation is already in a repo of its own. To bring it into Capella, we need to:

1. Create a capella branch in docs-analytics. Use feature flags to hide the REST API specific stuff until the Data API or Management API can handle them.
2. Add the docs-analytics navigation partial to the nav on the Capella side, and combine it with the Capella-specific UI docs.