Makefile: remove install-all target
remove obsolete manual

The new manual will contain text copied from Git's documentation.

The old manual was licensed under the FDL v1.2, while Git's
documentation is licensed under the GPL v2.  Being able to seed Magit's
documentation with pieces from Git is more valuable than keeping pieces
from Magit's old documentation (most of it would have to be discarded
anyway) and so the latter has to go.
README.md: indent code blocks

Don't use ```language ...```.  That looks worse in plain text
and while it used to look better on Github, it no longer does.

See http://www.greghendershott.com/2014/11/github-dropped-pygments.html.