doc: Update documentation to describe use of find_package(Zephyr)

This commit changes the current boilerplate include description and
instead describes the use of find_package(Zephyr)

It also add a section covering additional possibilities when using
find_package(Zephyr).

- Search order
- Zephyr repository application description
- Zephyr workspace application description
- Zephyr freestanding application description
- Environment ZEPHYR_BASE setting
- Multiple Zephyr and preference setting

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>