Improve description and preferences

[gbraad]

Replaces #86

Minor improvements to the extension description and preferences:

• Updated extension description. It is a bit bland, but shorter/concise and matches other extensions more.
• Used markdown to change link to pull secret into a real link. Easier to copy for now; will be clickable once [1] is fixed.

[1] https://github.com/containers/podman-desktop/issues/2230

[gbraad]

@deboer-tim Merged without the minimum memory in package.json (as described in #86 this conflicts with our preset validation logic).