Migrate documentation of capture agent communication protocol to markdown

When we were integrating our capture agent (currently working with Opencast 2.x), we used the documentation found on the old developer wiki: https://opencast.jira.com/wiki/display/MH/Capture+Agent+Communication+Protocols

That documentation was actually quite useful and seems missing in the official Opencast documentation.

The goal of this task is to migrate that documentation to the official Opencast documentation.