Document this new build system feature.
Since its purpose is customizing application builds, the logical place
for the main body of documentation is in a new snippets/ directory in
doc/build/. Create that directory and add its initial documentation.
Like boards and samples, however, we expect people to write
documentation for each snippet within the directory that defines the
snippet itself. Therefore, add a new top-level snippets/ directory and
stub out the documentation needed to document individual snippets as
well.
Add documentation and cross-references in other required places as
well.
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>
