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
• [x] I ran make check without errors
• [x] 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 ...

[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?