chore(docs): automate ReadTheDocs build trigger

[ncarenton]

Description

ReadTheDocs build is currently triggered manually. Automate it so documentation is rebuilt on every merge to main and on release.

Tasks

• Configure RTD incoming webhook on the GitHub repo (or validate it is active)
• Optionally trigger RTD build via API in the release workflow
• Auto-update release version in docs/conf.py during release

Context

Related to #69 / PR #215.