docs: add a FAQ

docs: add a FAQ

Git is an enormously flexible and powerful piece of software.  However,
it can be intimidating for many users and there are a set of common
questions that users often ask.  While we already have some new user
documentation, it's worth adding a FAQ to address common questions that
users often have.  Even though some of this is addressed elsewhere in
the documentation, experience has shown that it is difficult for users
to find, so a centralized location is helpful.

Add such a FAQ and fill it with some common questions and answers.
While there are few entries now, we can expand it in the future to cover
more things as we find new questions that users have.  Let's also add
section markers so that people answering questions can directly link
users to the proper answer.

The FAQ also addresses common configuration questions that apply not
only to Git as an independent piece of software but also the ecosystem
of CI tools and hosting providers that people use, since these are the
source of common questions.  An attempt has been made to avoid
mentioning any particular provider or tool, but to nevertheless cover
common configurations that apply to a wide variety of such tools.

Note that the long lines for certain questions are required, since
Asciidoctor does not permit broken lines there.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>