Update non-git code host, src-expose, and Perforce docs

Created by: dadlerj

Completely changed our docs for src-expose and non-git code hosts to emphasize one specific setup, and to better clarify .

That is, to use:

• git p4 clone and git p4 sync (and equivalent for other non-Git VCSs) to preserve change history.
• src-expose serve to serve them to Sourcegraph.

This intentionally discourages a few other paths, namely:

• Using src-expose (without the serve argument)... I'm not sure how big a codebase has to be before using git p4 sync is too slow after the initial clone, but I'll be getting a data point on this shortly from https://app.hubspot.com/contacts/2762526/company/693777200 who are going through this setup now.
• Cloning a non-Git project and pushing changes to a Git code host as a workaround.

An additional goal would be to use this to point customers to, to solve issues such as https://github.com/sourcegraph/customer/issues/39