Add info for how to get developer permissions

[jcfreeman2]

Pengfei brought this up today in the software coordinators channel on Slack. His suggestion: "add a “onboarding” page into our documentation, documenting the steps of getting added to the org and teams, GitHub username/displayed username conventions etc." We could simply add a sentence about this during the introductory paragraphs at the top of https://dune-daq-sw.readthedocs.io/en/latest/, and then link users to a page which expands on how to do this.

[jcfreeman2]

Thinking about this a little bit more, it seems like this material would logically be grouped with some of the topics in the https://dune-daq-sw.readthedocs.io/en/latest/packages/daq-release/ documentation - e.g., as a link under "For DAQ Software Developers and Users". In fact, this could also be an argument for moving material such as "DAQ software development workflow -- Gitflow" out of the daq-release documentation and into a general section like I describe above, since the git flow procedure doesn't really have anything to do with the daq-release package despite how useful it is to know this info. @dingp @alessandrothea let me know if you have any thoughts.

[andrewmogan]

There's an ongoing PR for this in daq-release#298. Further discussion should be done there.