Added async docker segment to display number of running, paused and …

[git-rz]

…exited containers.

Also added some commentary on how to add a new async segment, because I really wish there were docs for that when I started.

[romkatv]

Thanks for the PR. I should have time to look into it tomorrow.

[romkatv]

I took a look at the code and I think it would require a different design to make it more flexible. It wouldn't be easy for you to implement (powerlevel10k is optimized for my own maintenance costs with no regard for long it would take anyone else to do the job), so I'll do it myself when I get the time.

[git-rz]

Indeed, there does seem to be some magic related to _CONTENT_EXPANSION that has totally eluded me. I also see that the icon customization should fully use the framework.

How different a design are you thinking? I see a pattern whereby many segments consist of subsegments, and the framework seems to have a developer-unfriendly approach to handling them (unless the dev is you :), as you just said.. ). If you're envisioning a sweeping update to improve that, then I'll gladly stay out of the way. Otherwise, I'm eager to learn and help finish this segment.