docs: fix install instructions (package not yet on PyPI)

[fabio-rovai]

docs: fix install instructions (package is not on PyPI yet)

Summary

The README's Quick Start tells users to run pip install gen-fraud-graph as the
primary install path, but that package is not published on PyPI yet, so the command
fails for anyone following the docs. This PR replaces the failing command with the
from-source install path the project already documents (uv pip install -e), adds a
plain-pip source variant, and adds an honest note that the PyPI release is still
pending. No code changes.

Reproduction (clean environment)

A fresh virtual environment with only PyPI configured cannot resolve the package:

$ uv venv --python 3.12 /tmp/clean
$ /tmp/clean/bin/python -m pip install gen-fraud-graph
ERROR: Could not find a version that satisfies the requirement gen-fraud-graph (from versions: none)
ERROR: No matching distribution found for gen-fraud-graph

The same resolution failure via uv:

$ uv pip install gen-fraud-graph
  × No solution found when resolving dependencies:
  ╰─▶ Because gen-fraud-graph was not found in the package registry and you
      require gen-fraud-graph, we can conclude that your requirements are
      unsatisfiable.

This is consistent with the repo state: release.yml publishes to PyPI on a GitHub
Release, and no 0.1.0 release/tag has been published yet, so there is no
distribution to install.

Changes

• README.md: the Installation section now leads with the from-source uv pip install -e '.[dev]'
  path, lists the optional extras (local, openai, all) as source installs, adds a
  plain-pip source variant, and carries a short note that pip install gen-fraud-graph
  will only work once the package is published. The PyPI badge is left in place (it is
  pre-provisioned for the planned release).
• CHANGELOG.md: one-line entry under [Unreleased] / Fixed.

Notes

• Minimal, docs-only, no behaviour change.
• Per CONTRIBUTING.md, this repo requires signing the Santander Contributor License
  Agreement (CLA) before merge. The CLA Assistant workflow (.github/workflows/cla.yml)
  records signatures under .cla-signatures/. I will sign the CLA when prompted on the PR.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[fabio-rovai]

I have read the CLA Document and I hereby sign the CLA

[fabio-rovai]

I have read the CLA Document and I hereby sign the CLA

[opensource-SantanderAI]

LGTM. Accurate docs-only fix: the package isn't on PyPI yet, so 'pip install gen-fraud-graph' fails; the README now leads with the from-source install (uv and plain pip variants) and notes the pending PyPI release. All checks green.