Added a virtual environment instruction

[AA1999]

When building the docs myself, I encounter the following issue:

Defaulting to user installation because normal site-packages is not writeable
Collecting sphinx==1.8.6
  Downloading Sphinx-1.8.6-py2.py3-none-any.whl (3.1 MB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 3.1/3.1 MB 802.9 kB/s eta 0:00:00
Collecting docutils<0.18
  Downloading docutils-0.17.1-py2.py3-none-any.whl (575 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 575.5/575.5 kB 876.2 kB/s eta 0:00:00
Requirement already satisfied: sphinx-sitemap>=2.1.0 in /home/arshia/.local/lib/python3.10/site-packages (from -r requirements.txt (line 3)) (2.2.0)
Requirement already satisfied: sphinxcontrib-spelling in /home/arshia/.local/lib/python3.10/site-packages (from -r requirements.txt (line 4)) (4.2.1)
Requirement already satisfied: sphinx-notfound-page in /home/arshia/.local/lib/python3.10/site-packages (from -r requirements.txt (line 5)) (0.8)
Requirement already satisfied: jinja2<=3.0.3 in /home/arshia/.local/lib/python3.10/site-packages (from -r requirements.txt (line 6)) (3.0.3)
Requirement already satisfied: requests>=2.0.0 in /home/arshia/.local/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (2.25.1)
Requirement already satisfied: babel!=2.0,>=1.3 in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (2.11.0)
Requirement already satisfied: imagesize in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (1.4.1)
Requirement already satisfied: packaging in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (23.0)
Requirement already satisfied: sphinxcontrib-websupport in /home/arshia/.local/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (1.2.4)
Requirement already satisfied: alabaster<0.8,>=0.7 in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (0.7.13)
Requirement already satisfied: Pygments>=2.0 in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (2.14.0)
Requirement already satisfied: six>=1.5 in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (1.16.0)
Requirement already satisfied: setuptools in /home/arshia/.local/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (58.2.0)
Requirement already satisfied: snowballstemmer>=1.1 in /usr/lib/python3.10/site-packages (from sphinx==1.8.6->-r requirements.txt (line 1)) (2.2.0)
Requirement already satisfied: PyEnchant>=1.6.5 in /home/arshia/.local/lib/python3.10/site-packages (from sphinxcontrib-spelling->-r requirements.txt (line 4)) (3.2.2)
Requirement already satisfied: MarkupSafe>=2.0 in /usr/lib/python3.10/site-packages (from jinja2<=3.0.3->-r requirements.txt (line 6)) (2.1.2)
Requirement already satisfied: pytz>=2015.7 in /home/arshia/.local/lib/python3.10/site-packages (from babel!=2.0,>=1.3->sphinx==1.8.6->-r requirements.txt (line 1)) (2021.3)
Requirement already satisfied: certifi>=2017.4.17 in /usr/lib/python3.10/site-packages (from requests>=2.0.0->sphinx==1.8.6->-r requirements.txt (line 1)) (2022.12.7)
Requirement already satisfied: idna<3,>=2.5 in /home/arshia/.local/lib/python3.10/site-packages (from requests>=2.0.0->sphinx==1.8.6->-r requirements.txt (line 1)) (2.10)
Requirement already satisfied: urllib3<1.27,>=1.21.1 in /usr/lib/python3.10/site-packages (from requests>=2.0.0->sphinx==1.8.6->-r requirements.txt (line 1)) (1.26.12)
Requirement already satisfied: chardet<5,>=3.0.2 in /home/arshia/.local/lib/python3.10/site-packages (from requests>=2.0.0->sphinx==1.8.6->-r requirements.txt (line 1)) (4.0.0)
Requirement already satisfied: sphinxcontrib-serializinghtml in /usr/lib/python3.10/site-packages (from sphinxcontrib-websupport->sphinx==1.8.6->-r requirements.txt (line 1)) (1.1.5)
Installing collected packages: docutils, sphinx
  Attempting uninstall: sphinx
    Found existing installation: Sphinx 5.1.1
    Uninstalling Sphinx-5.1.1:
      Successfully uninstalled Sphinx-5.1.1
ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
sphinx-automodapi 0.14.1 requires sphinx>=2, but you have sphinx 1.8.6 which is incompatible.
Successfully installed docutils-0.17.1 sphinx-1.8.6

And the only way I found to fix it was to create a virtual environment and install the requirements there. This would help anyone facing issues like this.
I haven't installed these packages separately before, so this seems to be a conflict with some internal packages.

[RubenRBS]

Hi! Thanks for taking the time to improve the docs :)
The changes look great, but I suggested a wording change just to make it clearer that it's an optional step.

Also, I think the target branch might not be the proper one, but noone to blame but us with that, it's not super clear where each kind of change is supposed to go. Right now we're trying to make develop2 the default branch (But failing for mysterious reasons!) so I think it would be great to at least have a copy of this PR for that branch too!

[AA1999]

I will also make a copy for develop2 branch as requested.

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