New readme section : Requirements to build html page in virtual environments

[antoinemeyer5]

Simple guide for indicate:

• how to set up from scratch a Python virtual environment locally on a machine,
• install in that virtual environment all the requirements for the wiki build and
• do the build in that "safe" environment

The aim is to help documentation creators to:

• be in a controlled environment where they can be sure of using the right tools, versions, etc,
• detect potential errors or warnings as early as possible

[ajpowelsnl]

I don't have a strong opinion about this PR, other than the security caveat about using the PyCharm IDE in certain work environments. In any case, the merits of this approach vs. base install of the requirements, iterating change quickly by simply opening newly built file in a browser, are unclear. Perhaps it makes more sense if one is using an IDE ...?

[ajpowelsnl]

CAVEAT: there might be limits on using this (PyCharm) IDE in certain environments.