Adding an official documentation of building Qt documentation from source

Right now, users are either referring to README in Qt source, i.e. this part:

Documentation
=============
 
After configuring and compiling Qt, building the documentation is possible by running
 
```
cmake --build . --target docs
```
 
After having built the documentation, you need to install it with the following
command:
 
```
cmake --build . --target install_docs
```
 
The documentation is installed in the path specified with the
configure argument `-docdir`.

Or to a Wiki page:
https://wiki.qt.io/Building_Qt_Documentation

The problems are:
1. They are purposing seemingly different approaches and can be confusing
2. Neither is that "official" or easy to find. I suppose doc.qt.io is the place to go when user wants to find something (especially, building something from source)?

So it may be better to integrate those 2 articles into doc.qt.io, explaining what options people can have when try to build Qt doc by themselves.