Move documentation to Read the Docs

Read the docs is used to host the documentation.
https://language-data.readthedocs.io/en/latest/

The following documentation has been moved,

* Introduction
* Using the PHP / Node.js libraries
* Adding new languages
* PHP API documentation

Updated the README.md to point to the new documentation.

Doxygen is used to pull out the PHPDoc comments into XML.
This is parsed via doxyphp2sphinx into Sphinx which is
then used by Read the docs to generate the documentation.

Read the docs has been configured to update the code documentation
under the docs/api folder automatically whenever a commit is made
so no manual work is needed.

Bug: T218639

docs/index.rst

@@ -0,0 +1,113 @@
+.. LanguageData documentation master file, created by
+   sphinx-quickstart on Thu Jan 30 13:47:31 2020.
+   You can adapt this file completely to your liking, but it should at least
+   contain the root `toctree` directive.
+
+CLDR based language data and utilities
+========================================
+
+This library contains language related data, and utility libraries written in PHP and Node.js to
+interact with that data.
+
+The language related data comprises of the following,
+
+1. The script in which a language is written
+2. The script code
+3. The language code
+4. The regions in which the language is spoken
+5. The autonym - language name written in its own script
+6. The directionality of the text
+
+This data is populated from the current version of
+`CLDR supplemental data <http://unicode.org/repos/cldr/trunk/common/supplemental/supplementalData.xml>`_.
+
+Using the PHP library
+----------------------------
+|php-build|
+
+.. |php-build| image:: https://github.com/Abijeet/language-data/workflows/PHP%20build/badge.svg
+		 :target: https://github.com/Abijeet/language-data/actions?query=workflow%3A%22PHP+build%22
+
+Installation
+^^^^^^^^^^^^^
+You can add this library to your project by running:
+
+.. code-block:: bash
+
+		composer install wikimedia/language-data
+
+Basic usage
+^^^^^^^^^^^^^
+The basic usage is like this:
+
+.. code-block:: php
+
+		<?php
+		use Wikimedia\LanguageData;
+
+		$languageData = LanguageData::get();
+		// Returns English
+		$languageData->getAutonym( 'en' );
+
+For a full list of methods see the documentation for the `LanguageData <api/languagedata.html>`_ class.
+
+Using the Node.js library
+------------------------------
+
+|npm| |npm-build|
+
+.. |npm| image:: https://img.shields.io/npm/v/@wikimedia/language-data.svg
+      :target: https://npmjs.com/package/@wikimedia/language-data
+
+
+.. |npm-build| image:: https://github.com/Abijeet/language-data/workflows/Node.js%20build/badge.svg
+      :target: https://github.com/Abijeet/language-data/actions?query=workflow%3A%22Node.js+build%22
+
+Installation
+^^^^^^^^^^^^^
+You can add this library to your project by running,
+
+.. code-block:: bash
+
+		npm i @wikimedia/language-data
+
+Basic usage
+^^^^^^^^^^^^^
+The basic usage is like this:
+
+.. code-block:: js
+
+		const languageData = require('@wikimedia/language-data');
+
+		// Returns English
+		languageData.getAutonym( 'en');
+
+The exposed methods are similar to the methods present in the PHP `LanguageData <api/languagedata.html>`_ class.
+
+Contribute
+----------
+
+- Issue Tracker: https://github.com/wikimedia/language-data/issues
+- Source Code: https://github.com/wikimedia/language-data
+
+Navigation
+==========
+
+.. toctree::
+    :maxdepth: 1
+    :caption: User Documentation
+
+		Adding new languages <user/adding_new_language.rst>
+
+.. toctree::
+    :maxdepth: 2
+    :caption: PHP API Documentation
+
+  	LanguageData class <api/languagedata.rst>
+
+
+Indices and tables
+==================
+
+* :ref:`genindex`
+* :ref:`search`