Set up Read the Docs hosting for this repo

[lmolkova]

The repo already contains all the in-tree docs infrastructure:

• .readthedocs.yml, docs/conf.py, docs/index.rst, docs/instrumentation/*.rst
• rtd-requirements.txt / docs-requirements.txt
• A CI docs job (tox -e docs) that builds the docs on every PR

What's missing is the Read the Docs project itself. https://opentelemetry-python-genai.readthedocs.io/ currently returns 404 — no RTD project is registered for this repo, so docs are never published.

This requires an action by someone with OpenTelemetry org admin access (outside the repo):

1. Create a Read the Docs project for open-telemetry/opentelemetry-python-genai (matching the existing slug opentelemetry-python-genai so existing links resolve).
2. Connect the GitHub webhook so pushes to main and tags trigger builds.
3. Verify the build succeeds and latest resolves.