Feature: Show keyboard shortcuts on settings page

## Feature Request
Wallabagger has two poorly documented keyboard shortcuts. Theses should be mentioned on the settings page. As the behavior is changing, depending if 'content fetching from browser' is activated by default for all domains or not, we should give a hint right below this line:

### if activated:
> keyboard shortcuts:
> Alt+W = save content from the browser to wallabag
> Alt+Shift+W = only send current URL to wallabag

### if not activated
> keyboard shortcuts:
> Alt+W = send current URL to wallabag (with popup/meta-data editor)
> Shift+Alt+W = send current URL to wallabag (without popup)

### read this
https://kdecherf.com/blog/2023/08/02/fetch-content-from-browser-on-demand-with-wallabagger/

## Additionally, it would be super great
if we could change the key shortcut right here, e.g. if it conflicts with other plugins.