enhance docs, depends on module lxml

[bootchk]

On newly installed Ubuntu 20.04, after:
- clone this repository
- install package python3-pip
- install python3-virtualenv

then:

source bootstrap.sh
./tools/build.sh GimpUI-3.0

gave:

   from lxml import etree
ModuleNotFoundError: No module named 'lxml'

Then:

pip install lxml
./tools/build.sh GimpUI-3.0

seems to get past the previous error.

[bootchk]

Also, on a clean install of Ubuntu 20.10, the module 'requests' does not seem to be installed, but is a direct dependency of pgi-docgen, see gen/modules.py line 12.

Maybe the documented list of dependencies did not work for me since I used pip --no-deps.
In that case, you will find more transitive dependencies, such as the modules urllib3 and chardet, which the module requests seems to depend on.
Possibly one of the other documented dependencies installs module requests when not using --no-deps.

At any rate, the module requests is a direct dependency.