Documentation: Improve "What do I need to know" section

[RemDelaporteMathurin]

In the What do I need to know section we write that a github account is required to benefit from the latest updates.

This is simply not true (anymore) since it is possible to run:

pip install git+https://github.com/festim-dev/festim@main

to install the main branch version of the code.

However, it is true that an account is required to open issues and to contribute to the code.

I would remove this sentence and specify that git/GitHub knowledge is required to contribute to the code.