Add/update existing user onboarding docs

Created by: erzhtor

Update existing docs pages to make them more organized and clear https://docs.sourcegraph.com/code_search (or create a separate).

Something like "Using Sourcegraph" group/section docs with two main user targets:

1. I'm user > teach how to use sourcegraph functionality
2. I'm admin > teach how to administer/manage instance

Follow-up on PD 31 doc