deploy docs to readthedocs

[samsammurphy]

What maintenance does this project need?

We can easily deploy docs using github pages

I think we need to use the repository settings (which I don't have access to) and, under the "GitHub Pages" section, select the gh-pages branch for GitHub Pages.

This is the default branch that mkdocs gh-deploy command will build and deploy to

Organisation

No response

[corentincarton]

Changing the title of the issue as we agreed to pull from readthedocs as ECWMF documentation is centralised there.