docs: keep top tabs always visible and hide footer

* per default, navigation tabs disappear when you scroll down the page,
  which may confuse some users
* hide "Made with Material" footer which is visible when scrolling to
  the bottom, which takes up space and may make the left menu scrollbar
  to appear, and thus reduce usability.

Author: ukkopahis

docs/style.css

@@ -0,0 +1,4 @@
+/* hide "Made with Material" footer */
+.md-footer-meta {
+    display: none;
+}

mkdocs.yml

@@ -20,6 +20,7 @@ theme:
   features:
     - tabs
     - navigation.tabs
+    - navigation.tabs.sticky
     - navigation.sections
 
 plugins:
@@ -48,6 +49,9 @@ plugins:
         Menu-System.md: Developers/Menu-System.md
         Contributing-Services.md: Developers/index.md
 
+extra_css:
+  - style.css
+
 markdown_extensions:
   - admonition
   - pymdownx.superfences