Add option to generate a sitemap

[mortenpi]

Just wanted to open a draft PR here with this naive implementation, which takes all the HTML files and generates the sitemap with all of them.

However, I realized that this way we'll end up with all the old versions also in the sitemap, and I don't this that's a good idea, since sitemaps should also point to canonical content. So we need to be a bit smarter about it.

We could try to interpret the file system paths, but I figured a better way is to re-use canonical URLs, which we can parse from the HTML. The sitemap will be just the canonical URLs, but we'll also make sure that the canonical URL is reasonable. HTML files with missing or broken canonical URLs get ignored.

This pushes the responsibility of having good canonical URLs to the HTML generation. So I think we want a post-processing step here as well for each subpage, to catch and/or fix bad or missing canonical URLs.

[mortenpi]

A few changes

• canonical -> canonical_domain, since you should only have the domain part of the URL there. The root portion of the path comes from rootpath.
• Adds sitemap::Bool and sitemap_filename (optional), to toggle the sitemap generation. Since the implementation is based on canonical URLs now, you need to set canonical_domain (and therefore enable canonical URL rewriting) when you set sitemap=true.
• I reorganized the runtests.jl a bit, moving the MultiDocumenter run out of the testset, so that it would be easier to rapidly re-run the test suite (when developing).
• It adds a couple of tests for canonical URLs as well.