Doc modernization (#3474)

* Change doc build to reST exclusively

* Rewrite Intro doc in reST; create toctree

* Update parameter and contribute

* Convert tutorials to reST

* Convert Python tutorials to reST

* Convert CLI and Julia docs to reST

* Enable markdown for R vignettes

* Done migrating to reST

* Add guzzle_sphinx_theme to requirements

* Add breathe to requirements

* Fix search bar

* Add link to user forum

doc/_static/custom.css

@@ -0,0 +1,23 @@
+div.breathe-sectiondef.container {
+  width: 100%;
+}
+
+div.literal-block-wrapper.container {
+  width: 100%;
+}
+
+.red {
+  color: red;
+}
+
+table {
+  border: 0;
+}
+
+td, th {
+  padding: 1px 8px 1px 5px;
+  border-top: 0;
+  border-bottom: 1px solid #aaa;
+  border-left: 0;
+  border-right: 0;
+}