Add XML sitemap

[gonX]

Fixes #323

Includes /sitemap.xml, as well as canonical links for all HTML pages using the default template.

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From Google documentation:

Pick a canonical URL for each of your pages and submit them in a sitemap. All pages listed in a sitemap are suggested as canonicals; Google will decide which pages (if any) are duplicates, based on similarity of content.

Supplying the preferred canonical URLs in the sitemaps is a simple way of defining canonicals for a large site, and sitemaps are a useful way to tell Google which pages you consider most important on your site.

Source: developers.google.com - Use a sitemap

Also listed as cons for the sitemap technique, signifying that we do also still want canonical HTML/HTTP elements:

• Google must still determine the associated duplicate for any canonicals that you declare in the sitemap.
• Less powerful signal to Google than the rel="canonical" mapping technique.

Source: developers.google.com - Comparison of canonicalization methods

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Generated sitemap containing all URL's of current site attached here: sitemap.xml