git-integration.txt: move into Documentation/

As more documentation options are added (e.g. for generating website
HTML) it will be better if documentation is isolated in a subdirectory.

Signed-off-by: John Keeping <john@keeping.me.uk>
Makefile: move documentation rules into Documentation/Makefile

Signed-off-by: John Keeping <john@keeping.me.uk>
Documentation: extract description from git-integration.txt

This will allow the description to be re-used on the web page.

Signed-off-by: John Keeping <john@keeping.me.uk>
README: extract install information into Documentation/

This cuts the README down to a simple "see these other places", and
allows the install information to be more easily used in a web page.

Signed-off-by: John Keeping <john@keeping.me.uk>
Add gh-pages target to the Makefile

This updates the documentation on the gh-pages branch from what is
currently in the repository.

Signed-off-by: John Keeping <john@keeping.me.uk>