Add "install requirements" in documentation

[DevBenLefevre]

Hello !

First : THANK YOU !

Then : I may have bad installed the projet, but I installed every dependencies in the dependencies section of the documentation one by one, following the links.

Since, I had
Traceback (most recent call last): File "/Users/b.lefevre/dev/ddt4all/main.py", line 15, in <module> import PyQt5.QtWebEngineWidgets as webkitwidgets ModuleNotFoundError: No module named 'PyQt5.QtWebEngineWidgets'
first running the project, I had to run python3 -m pip install -r requirements.txt on MacOS.

I think it can be a good idea to add it to the documentation, probably in dependencies section.

[Furtif]

read #320 (comment)

[Furtif]

or #1112