Add explicit permalink/canonical url to front matter

[lidel]

As per verbal with @hacdias we want to have explicit URL of the spec in the front matter, and then have generator use the URL.pathname during generation, instead of the directory name.

Goal is to ensure spec URLs never change https://www.w3.org/Provider/Style/URI

[hacdias]

We can already do that, but we cannot include the domain name AFAIK: https://www.11ty.dev/docs/permalinks/. I see two options:

a. Use Eleventy's default to only put the path.
b. Try to customize spec-generator such that it is able of stripping the domain from the permalink.

In either case we probably want spec-generator to scream at you if it's missing in order to enforce explicit permalinks. I don't necessarily see the benefit of including the domain on the frontmatter.