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

.gitignore

@@ -15,4 +15,10 @@ yarn-error.log*
 *.sln
 
 # PHP
-vendor/
+vendor/
+
+# Docs
+docs/_build/
+docs/warnings.log
+docs/xml/
+docs/html/