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.

docs/CONTRIBUTING.md

@@ -0,0 +1,121 @@
+Submitting patches
+------------------
+
+You can:
+
+ 1. Send mail to ostree-list@gnome.org, with the patch attached
+ 1. Submit a pull request against https://github.com/GNOME/ostree
+ 1. Attach them to https://bugzilla.gnome.org/
+
+Please look at "git log" and match the commit log style.
+
+Running the test suite
+----------------------
+
+Currently, ostree uses https://wiki.gnome.org/GnomeGoals/InstalledTests
+To run just ostree's tests:
+
+    ./configure ... --enable-installed-tests
+    gnome-desktop-testing-runner -p 0 ostree/
+
+Also, there is a regular:
+
+    make check
+
+That runs a different set of tests.
+
+Coding style
+------------
+
+Indentation is GNU.  Files should start with the appropriate mode lines.
+
+Use GCC `__attribute__((cleanup))` wherever possible.  If interacting
+with a third party library, try defining local cleanup macros.
+
+Use GError and GCancellable where appropriate.
+
+Prefer returning `gboolean` to signal success/failure, and have output
+values as parameters.
+
+Prefer linear control flow inside functions (aside from standard
+loops).  In other words, avoid "early exits" or use of `goto` besides
+`goto out;`.
+
+This is an example of an "early exit":
+
+    static gboolean
+    myfunc (...)
+    {
+        gboolean ret = FALSE;
+    
+        /* some code */
+    
+        /* some more code */
+    
+        if (condition)
+          return FALSE;
+    
+        /* some more code */
+    
+        ret = TRUE;
+      out:
+        return ret;
+    }
+
+If you must shortcut, use:
+
+    if (condition)
+      {
+        ret = TRUE;
+        goto out;
+      }
+
+A consequence of this restriction is that you are encouraged to avoid
+deep nesting of loops or conditionals.  Create internal static helper
+functions, particularly inside loops.  For example, rather than:
+
+    while (condition)
+      {
+        /* some code */
+        if (condition)
+          {
+             for (i = 0; i < somevalue; i++)
+               {
+                  if (condition)
+                    {
+                      /* deeply nested code */
+                    }
+    
+                    /* more nested code */
+               }
+          }
+      }
+
+Instead do this:
+    
+    static gboolean
+    helperfunc (..., GError **error)
+    {
+      if (condition)
+       {
+         /* deeply nested code */
+       }
+    
+      /* more nested code */
+    
+      return ret;
+    }
+    
+    while (condition)
+      {
+        /* some code */
+        if (!condition)
+          continue;
+    
+        for (i = 0; i < somevalue; i++)
+          {
+            if (!helperfunc (..., i, error))
+              goto out;
+          }
+      }
+