Add some docs to talk about app versioning

Right now there seems to be not much saying what the `ApplicationVersion` and `ApplicationDisplayVersion` properties do and how they work for a cross-platform level.

In addition to this, these numbers are retrieved using maui essentials, so that may be useful.

Also, Windows is special... They do not support the 2 system, so we "fake" it by using the 4 part version for windows:

Full app version = `"$(ApplicationDisplayVersion).$(ApplicationVersion)`

We then split it up later in essentials. Nasty but it is all we have.

Use the ApplicationVersion and ApplicationDisplayVersion properties named in the issue as the documentation scope, including how MAUI Essentials retrieves them. Document the cross-platform behavior and the Windows four-part version workaround, then check the existing documentation for the right place and verify that the explanation covers the stated full app version format.

Written by the indexing model from the issue text.