Add openqa-cli documentation

[kraih]

So far we don't have much client documentation, even though most openQA users will interact with it directly. This is my first attempt at collecting everything i could think of and turning it into a new guide. Still needs some cleanups and perhaps more diverse examples, but the main points are there.

Progress: https://progress.opensuse.org/issues/10316
Feature PR: #2859

[kraih]

I did not touch the other guides yet (which also contain some small client sections). And i'm not sure where in the overall documentation it belongs. Placing it above Pitfalls is just a starting point.

[codecov]

Codecov Report

Merging #3009 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3009      +/-   ##
==========================================
+ Coverage   91.88%   91.89%   +0.01%     
==========================================
  Files         207      207              
  Lines       12835    12835              
==========================================
+ Hits        11793    11795       +2     
+ Misses       1042     1040       -2     

Impacted Files	Coverage Δ
lib/OpenQA/Worker/CommandHandler.pm	93.75% <0.00%> (+1.56%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 647c61d...901ee2b. Read the comment docs.

[kalikiana]

I feel like openqa.opensuse.org might be a better default if you just wanna try and look at the potential use cases. 🤔 Although I gather this could be construed as "too specific to opensuse" somehow.

[kraih]

You also don't want random people cause load on O3 with test requests, or accidentally pasted destructive examples being run against O3 by people who have the credentials to do damage. 😉

[okurz]

Overall this reads like a very nicely written guide and I think it can be helpful and also motivate more users to try out the API when previously maybe they only used the web UI. I think the current level of detail you hit is exactly right. We have only a tiny overlap which I think you handle well with "You have seen a few examples earlier", etc. :)