submodules: overhaul documentation

submodules: overhaul documentation

This patch aims to detangle (a) the usage of `git-submodule`
from (b) the concept of submodules and (c) how the actual
implementation looks like, such as where they are configured
and (d) what the best practices are.

To do so, move the conceptual parts of the 'git-submodule'
man page to a new man page gitsubmodules(7). This new page
is just like gitmodules(5), gitattributes(5), gitcredentials(7),
gitnamespaces(7), gittutorial(7), which introduce a concept
rather than explaining a specific command.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>