Doc modernization (dmlc#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

Author: hcho3

doc/R-package/index.md

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+#################
+XGBoost R Package
+#################
+
+.. raw:: html
+
+  <a href="http://cran.r-project.org/web/packages/xgboost"><img alt="CRAN Status Badge" src="http://www.r-pkg.org/badges/version/xgboost"></a>
+  <a href="http://cran.rstudio.com/web/packages/xgboost/index.html"><img alt="CRAN Downloads" src="http://cranlogs.r-pkg.org/badges/xgboost"></a>
+
+You have found the XGBoost R Package!
+
+***********
+Get Started
+***********
+* Checkout the :doc:`Installation Guide </build>` contains instructions to install xgboost, and :doc:`Tutorials </tutorials/index>` for examples on how to use XGBoost for various tasks.
+* Read the `API documentation <https://cran.r-project.org/web/packages/xgboost/xgboost.pdf>`_.
+* Please visit `Walk-through Examples <https://github.com/dmlc/xgboost/tree/master/R-package/demo>`_.
+
+*********
+Tutorials
+*********
+
+.. toctree::
+  :maxdepth: 2
+  :titlesonly:
+  
+  Introduction to XGBoost in R <xgboostPresentation>
+  Understanding your dataset with XGBoost <discoverYourData>

doc/R-package/index.rst

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+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;
+}