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/index.rst

@@ -0,0 +1,30 @@
+#####################
+XGBoost Documentation
+#####################
+
+**XGBoost** is an optimized distributed gradient boosting library designed to be highly **efficient**, **flexible** and **portable**.
+It implements machine learning algorithms under the `Gradient Boosting <https://en.wikipedia.org/wiki/Gradient_boosting>`_ framework.
+XGBoost provides a parallel tree boosting (also known as GBDT, GBM) that solve many data science problems in a fast and accurate way.
+The same code runs on major distributed environment (Hadoop, SGE, MPI) and can solve problems beyond billions of examples.
+
+********
+Contents
+********
+
+.. toctree::
+  :maxdepth: 2
+  :titlesonly:
+
+  build
+  get_started
+  tutorials/index
+  faq
+  XGBoost User Forum <https://discuss.xgboost.ai>
+  GPU support <gpu/index>
+  parameter
+  Python package <python/index>
+  R package <R-package/index>
+  JVM package <jvm/index>
+  Julia package <julia>
+  CLI interface <cli>
+  contribute