Improve the README, and various related changes

[Marc-Andrieu]

Description

Summary

See my memo at #925, the goal of this PR is to resolve the heap of things in this issue.

Issues/PR dependencies

Issues to be resolved

Fixes #925

Required PRs

Changes Made

• CoreUsersFactory
  • Valid ASCII emails
  • Catch collisions
  • Don't crash if exactly 1 demo user
• ...

Additional Notes

Classification

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 🔨 Refactor (non-breaking change that neither fixes a bug nor adds a feature)
• 🔧 Infra CI/CD (changes to configs of workflows)
• 💥 BREAKING CHANGE (fix or feature that require a new minimal version of the front-end)
• 😶‍🌫️ No impact for the end-users

Impact & Scope

• Core functionality changes
• Single module changes
• Multiple modules changes
• Database migrations required
• Other: see Improve the README #925

Testing

• 1. Tested this locally
• 2. Added/modified tests that pass the CI (or tested in a downstream fork)
• 3. Tested in a deployed pre-prod
• 0. Untestable (exceptionally), will be tested in prod directly

Documentation

• Updated the docs accordingly :
• " Docstrings
• # Inline comments
• No documentation needed

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.90%. Comparing base (457e7ae) to head (13d07b6).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
app/core/users/factory_users.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #926      +/-   ##
==========================================
- Coverage   84.91%   84.90%   -0.01%     
==========================================
  Files         199      199              
  Lines       14166    14170       +4     
==========================================
+ Hits        12029    12031       +2     
- Misses       2137     2139       +2     

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