docs(api): Add python venv as a pre-requisite for API local dev

[tushar5526]

setup

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• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Contributes to

Mention that a user needs to have a virtual environment created and activated. Most new distro releases uphelds PEP 668 and prevents users from globally installing python packages by default.

Please describe.

How did you test this code?

On Ubuntu Noble:

sudo apt install python3.12-venv
git clone https://github.com/Flagsmith/flagsmith.git
cd flagsmith
python3 -m venv .venv
source .venv/bin/activate
make install

Please describe.

[vercel]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (4ffbf74) to head (1cdce17).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7722      +/-   ##
==========================================
- Coverage   98.52%   98.37%   -0.16%     
==========================================
  Files        1445     1445              
  Lines       55083    55079       -4     
==========================================
- Hits        54273    54185      -88     
- Misses        810      894      +84     

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