Minimal docs at module level

[peso]

This Pull Request provides minimal top-level documentation.
This makes the initial navigation of cargo docs easier.

It changes the following:

• add module comment to main
• add module comment to tabs
• add module comment to cmdbar
• add module comment to components
• add module comment to components::commitlist
• add crate comments to asyncgit

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[peso]

Next time, I should remember running rustfmt and cargo clippy before pushing ...

[extrawurst]

Lets remove commitlist and cmdbar mod comments as it feels arbitrary to only add them for these.

In general I dont want to list stuff that will easily get outdated like in your main.rs comment.

I prefer a mod comment over a parent mod comment explaining its sub-mods

[peso]

Removed commitlist and cmdbar mod coments.
I have tried to make main.rs more generic while still giving enough information that you don't have to read every module doc before knowing which is relevant to you.

[peso]

github says there is one more requested change, but I think I did address them all.
Did I miss one?