Documentation: Refactor and switch to Sphinx

* convert documentation to reStructuredText
* switch from mkdocs to sphinx
* rewrite README.md (now .rst) to include less information and be more
attractive

doc/_static/css/restic.css

@@ -0,0 +1,10 @@
+@import url('theme.css');
+
+.wy-side-nav-search {
+  background-color: #0000b4;
+}
+
+.logo {
+  height: 50% !important;
+  width: 50% !important;
+}