Specify site_url in MkDocs configuration for correct sitemap

[StevenMaude]

Fixes #1053.

To correctly generate the sitemap.xml, the site_url must be specified in mkdocs.yml.

Having a valid sitemap can help search engines index a site's content.

What I've done here is to make site_url switchable depending on an environment variable, MKDOCS_SITE_URL and then simply set that on build. As this is only really required for deployment, I've just put this in the deployment workflow, instead of adding some wrapper or to do the build with the environment variable supplied.

This keeps the local development setup exactly as it was, with no value set for site_url.

I'm reasonably convinced that this is the right approach because:

• we can override the site_url
  • to set as an empty string if we ever want to build a local file distribution (this may not be necessary with a missing value, but not tested)
  • when running locally with no URL set, the configuration is as it was previously (although I think with mkdocs serve, the site URL is ignored anyway!)
• we probably do want the canonical URL to be https://docs.opensafely.org/some-path… even for Cloudflare test deployments, technically, should a search engine somehow stumble upon those pages

[StevenMaude]

I'm reasonably convinced that this is the right approach…

…but am happy to be persuaded otherwise, that, for instance, we should only set site_url for actual production deployments to the site (and not for other Cloudflare Pages previews that we get)

[inglesp]

Looks reasonable.