Improve docs on accessing airlock

[rebkwok]

How to access Airlock is not very clear from the docs. This is exacerbated by the fact that
a) you have to go to job-server to get your token to log in
b) you can also view L4 output files on job-server

In addition, we've been assuming that users go to job-server outside L4, THEN log into L4 and go to airlock. But actually, it's quite possible to log into L4, go to job-server and get a token INSIDE L4, and then users need to navigate to the Airlock URL. That's confusing, especially if they've previously viewed output files on job-server. (See this thread.)

The accessing airlock docs do state where to go to get to Airlock. We probably want to highlight that in a box at the top of the page, and maybe add an explicit note that Airlock is a different website to job-server.

Currently, the Airlock docs are their own section in the releasing from L4 docs, because the old process is still being used. Until we can make the docs only refer to Airlock, it would be helpful to add a "go here if you're using Airlock" link to all of the old process pages.

[inglesp]

Covered by #444.