[docs] documentation improvement (#976)

* fixed typos and hotfixes

* converted gcc-tips.Rmd; added ref to gcc-tips

* renamed files

* renamed Advanced-Topics

* renamed README

* renamed Parameters-Tuning

* renamed FAQ

* fixed refs to FAQ

* fixed undecodable source characters

* renamed Features

* renamed Quick-Start

* fixed undecodable source characters in Features

* renamed Python-Intro

* renamed GPU-Tutorial

* renamed GPU-Windows

* fixed markdown

* fixed undecodable source characters in GPU-Windows

* renamed Parameters

* fixed markdown

* removed recommonmark dependence

* hotfixes

* added anchors to links

* fixed 404

* fixed typos

* added more anchors

* removed sphinxcontrib-napoleon dependence

* removed outdated line in Travis config

* fixed max-width of the ReadTheDocs theme

* added horizontal align to images

docs/requirements.txt

-sphinx<=1.5.6
-sphinxcontrib-napoleon
-sphinx_rtd_theme
-recommonmark

python-package/README.rst

@@ -88,7 +88,7 @@ Refer to the walk through examples in `Python guide folder <https://github.com/M
 Troubleshooting
 ---------------
 
-Refer to `FAQ <https://github.com/Microsoft/LightGBM/tree/master/docs/FAQ.md>`_.
+Refer to `FAQ <https://github.com/Microsoft/LightGBM/tree/master/docs/FAQ.rst>`_.
 
 Developments
 ------------