Doc: add getting-started tutorial

[tang-mm]

Description

Added a basic tutorial for first-time end users, covering:

• installing the CLI via Snap
• configure the default server
• creating a first job
• submit job
• check job status
• check job output

(This tutorial has some overlap with the how-to guides in #143 , with fewer options and more explanation for new users.)

Resolved issues

Resolves CERTTF-222

Documentation

Preview version on Readthedocs:

https://canonical-testflinger.readthedocs-hosted.com/en/doc-preview/tutorial

Tests

To check the documentation on a local sphinx site:

$ cd testflinger/docs
$ make install
$ make run     # Serve docs on http://127.0.0.1:8000/reference/test-phases/

[plars]

Those preview links are super-helpful, thanks for including those!
This looks good other than some strange syntax highlighting. I would guess it could either be turned off for those code blocks or changed to some other type of highlighting.

[plars]

It looks like the earlier section for testflinger-cli looks fine for me now since you removed "bash", but the later one still had weird highlighting.

I found that if I use "text" instead, then it renders fine for me now. I hope you don't mind, I just pushed the change, since it was trivial. And I'm +1 on it now. But I'll let you make the final call :)

[kevinyehk]

LGTM +1