Documentation update

[pgajek2]

Is your feature request related to a problem? Please describe.

1. Add search capability to the documentation so user doesn't have to scroll to find.
2. Add more examples comparing the standard way vs async way. eg. Database.Stateful - is it work the same? How to use queryLocator? Should we call it directly in the work() method?
3. At the beginning of each API page: Queueable, Schedulable, Batchable there should be an example of class, so I don't need to go to open https://async.beyondthecloud.dev/getting-started.html#_1-queueablejob-base-class to understand the concept.