docs: Setup mkdocs with mkdocs-material theme

This changes the framework used to generate the klipper3d site.

Signed-off-by: Damien Martin <damlobster@gmail.com>

docs/_klipper3d/css/extra.css

@@ -0,0 +1,15 @@
+[data-md-color-scheme="slate"] {
+    --md-primary-fg-color: hsla(var(--md-hue),15%,12%,1);
+    --md-default-bg-color: hsla(var(--md-hue),17%,17%,1);
+    --md-typeset-a-color:  steelblue;
+    --md-accent-fg-color:  lightblue;
+}
+
+img {
+    background-color: white;
+}
+
+.center-image {
+    margin: 0 auto;
+    display: block;
+}