jekyll: create plugin to update sitemap

[crazy-max]

follow-up #14688

In the same spirit as the plugin to fetch remote resources, create a new one to update our sitemap.

Also enables sitemap generation in development so we can preview sitemap changes. Since Jekyll 4, sitemap generation is pretty well optimized and takes less than 3 seconds for our docs so I think we can enable it.

I also added another commit which adds a sitemap xsl, so we can easily browse our sitemap:

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[crazy-max]

Here is the preview link for the sitemap: https://deploy-preview-14784--docsdocker.netlify.app/sitemap.xml

[thaJeztah]

Let me check this one; I think the intent here was to remove some hard-coded URLs in the docs (e.g. from upstream MarkDown files) that have a full https://docs.docker.com/ URL in them

URLs in the pages generated by Jekyll are expected to all be relative (so <a href=/foo/bar/>, without a domain); the intent of that is so that the generated pages can be deployed to "any" domain (staging/local/elsewhere) without links going to the "live" domain.

So perhaps that part was intentionally set to docs.docker.com

As to the sitemap.xml having docs.docker.com hard-coded; that may also have been intentional at the time, as we've had cases where Google found preview sites, and (although we set noindex headers), we wanted to avoid it finding the sitemap.xml, and as a result indexing those preview domains (and as a "best effort", we just pointed it to the live website, which was the "least bad" option.