Documentation workflow: auto-publish from master to gh-pages

[Guts]

Documentation is stored in a separate branch, which is the published one as static website on GitHub Pages.

The goal of this PR is to store the documentation into the docs folder and to automatically generate and deploy to gh-pages through a GitHub Action.

Advantages:

• embed other files (changelog, main readme...)
• encourage to update the docs during development

[Guts]

New doc is ready. For safety, I've duplicated existing gh-pages content to https://github.com/QGIS-Contribution/QGIS-ResourceSharing/tree/gh-pages-backup.

Here is a snapshot of how the new doc looks: