Deploy docs with GitHub Actions

[hdigital]

Add the built docs site to the repo and deploy docs with GitHub Actions.

• [x] add docs build to repo
• [x] configure GitHub Actions deploy static page
• [x] check successful deploy

Previous workflow based on mkdocs gh-deploy

[hdigital]

successful deploy

https://github.com/hdigital/parlgov-web/actions/runs/11048762382