[QA] beta tester docs update

[jnweiger]

WHERE Does This Need To Be Documented (Link)?

https://doc.owncloud.com/server/developer_manual/testing/test-pilots.html#why-do-you-want-to-join

WHY Should This Change Be Made?

This page needs a bigger review.

• The list of active contributors on owncloud.org (or owncloud.com) does not exist, afaik.
• The instructions "How do you test" do not match the way first time community contributors would work.
  • It should talk about explorative testing while using (testing on a dedicated test setup is unrealistic for starters)
  • It should invite to report a broader range of things (all regressions(!), broken links, outdated docs, papercuts, confusing/contradicting dialogs/instructions, missing translations)
  • it should explain, how we work (to clarify which type of reports have a lower chance of being addressed, feature requests, individual config, anything that was reported before)
  • it should not ask for "Check if what you found is a genuine bug"
• maybe PR #3844