Add documentation with MkDocs to VAPython

This MR add a documentation site to VAPython. For this it uses MkDocs and MkDocstrings. The docs for the generated VA interface are stored in a separate toml file. Maybe in the future, this doc file can be used as a unified documentation for all modules that implement some sort of VA interface.

docs/css/custom.css

+/* Indentation. */
+div.doc-contents:not(.first) {
+    padding-left: 25px;
+    border-left: .05rem solid var(--md-typeset-table-color);
+  }
\ No newline at end of file