Rewrite manual in mkdocs

I don't much like Docbook (and am considering converting the man pages
too), but let's start with the manual.

I looked at various documentation generators (there are a lot), and
I had a few requirements:

 - Markdown
 - Packaged in Fedora
 - Suitable for upload to a static webserver

`mkdocs` seems to fit the bill.

CONTRIBUTING.md

@@ -0,0 +1 @@
+docs/CONTRIBUTING.md

README.md

@@ -46,8 +46,8 @@ versions support extended validation using
 However, in order to build from a git clone, you must update the
 submodules.  If you're packaging OSTree and want a tarball, I
 recommend using a "recursive git archive" script.  There are several
-available online; [this
-code](https://git.gnome.org/browse/ostree/tree/packaging/Makefile.dist-packaging#n11)
+available online;
+[this code](https://git.gnome.org/browse/ostree/tree/packaging/Makefile.dist-packaging#n11)
 in OSTree is an example.
 
 Once you have a git clone or recursive archive, building is the

apidoc/Makefile.am

@@ -95,11 +95,6 @@ HTML_IMAGES=
 # Extra SGML files that are included by $(DOC_MAIN_SGML_FILE).
 # e.g. content_files=running.sgml building.sgml changes-2.0.sgml
 content_files= \
-	overview.xml \
-	repo.xml \
-	deployment.xml \
-	atomic-upgrades.xml \
-	adapting-existing.xml \
 	$(NULL)
 
 # SGML files where gtk-doc abbrevations (#GtkWidget) are expanded