Revise BBS documentation to mention webhooks and give clearer instructions

Created by: mrnugget

This is just the first step. I plan on adding more docs about the webhook integration after today's sync with @eseliger.

But what this does is:

• Consistently name it Sourcegraph Bitbucket Server plugin instead of mixing that and Sourcegraph native code intelligence plugin
• Mention the features of the plugin
• Give clearer and easy to find instructions on how to install and set it up

All of this can be improved and I will probably push to this PR.