Run `mkdocs` in merge request CI runs and upload the generated website
as artifacts.

To preview the documentation website in a merge request, click on
1. The pipeline link (`Merge request pipeline #1234 ...`)
2. `mkdocs` (under `quality check`)
3. `Browse` (under `Job artifacts` on the right)
4. `docs_site` -> `index.html`

Note: GitLab's built-in artifact browser is able to handle direct HTML
links (e.g. `Prerequisites.html`), but it does not support redirecting
directory links (e.g. `Prerequisites/` -> `Prerequisites/index.html`),
so we set `use_directory_urls: false` in `mkdocs.yml` to use page HTML
URLs in favor of directory URLs.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>