Document installation of requirements.txt with pip

Also pin their versions to the same ones as used on RTD.

Follow-up to #3276.

README.md

@@ -43,12 +43,12 @@ Similarly, you can include attachments (like assets as support material for a tu
 ## Building with Sphinx
 
 To build the HTML website (or any other format supported by Sphinx, like PDF, EPUB or LaTeX), you need to install [Sphinx](https://www.sphinx-doc.org/) >= 1.3 as well as (for the HTML) the [readthedocs.org theme](https://github.com/snide/sphinx_rtd_theme). Only the Python 3 flavor was tested, though the Python 2 versions might work too.
+You also need to install the Sphinx extensions defined in `requirements.txt`.
 
 Those tools are best installed using [pip](https://pip.pypa.io), Python's module installer. The Python 3 version might be provided (on Linux distros) as `pip3` or `python3-pip`. You can then run:
 
 ```sh
-pip3 install sphinx
-pip3 install sphinx_rtd_theme
+pip install -r requirements.txt
 ```
 
 You can then build the HTML documentation from the root folder of this repository with:

requirements.txt

@@ -1,5 +1,13 @@
-# Code tabs for GDScript/C#
-sphinx-tabs
+# Base dependencies
+
+# Sync with readthedocs:
+# https://github.com/readthedocs/readthedocs.org/blob/master/requirements/pip.txt
+# https://github.com/readthedocs/readthedocs.org/blob/master/requirements/local-docs-build.txt
+sphinx==1.8.5  # pyup: <2.0.0
+sphinx_rtd_theme==0.4.3
+
+# Code tabs extension for GDScript/C#
+sphinx-tabs==1.1.13
 
 # Full-page search UI for RTD: https://readthedocs-sphinx-search.readthedocs.io
 git+https://github.com/readthedocs/readthedocs-sphinx-search@master