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/tutorials/index.rst

@@ -0,0 +1,19 @@
+#################
+XGBoost Tutorials
+#################
+
+This section contains official tutorials inside XGBoost package.
+See `Awesome XGBoost <https://github.com/dmlc/xgboost/tree/master/demo>`_ for more resources.
+
+.. toctree::
+  :maxdepth: 1
+  :caption: Contents:
+
+  model
+  aws_yarn
+  dart
+  monotonic
+  input_format
+  param_tuning
+  external_memory
+