build/development.7: System building examples

Consolidate examples to build the system in the build(7) manual where
the details about the syntax involved is documented. Remove them from
development(7), instead adding examples for patching the trees. They
are left in UPDATING for now, to hopefully be removed eventually.

Reduce documentation proliferation and create consistent, maintainable
doc about building the system, eventually removing the COMMMON ITEMS
section of UPDATING so there is a single source of truth in the cannon
and well-integrated location, the in-band system reference manual.

+ Example 1: Build and upgrade system in place
+ Example 2: Build and upgrade a custom kernel in place
+ Example 3: Build and upgrade a single piece of userspace
+ Example 4: Build and upgrade a loadable kernel module
+ Example 5: Quickly rebuild a kernel in place
+ Example 6: Cross-compiling for different architectures

The examples section of development(7) now contains examples for
applying patches to the trees.

There is certainy much more to do here, but it needs to be chunked.

MFC after: 3 days
Reviewed by: imp (previous version), ivy (previous version)
Discussed with: emaste, imp, jhb, ivy
Differential Revision: https://reviews.freebsd.org/D48693