Add sitemap.xml to improve indexing by search engines

[BinderDavid]

Fixes #537 (hopefully)

The generated sitemap looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">

    <url>
        <loc>https://errors.haskell.org/messages/Cabal-7070/index.html</loc>
        <changefreq>weekly</changefreq>
        <lastmod>2024-10-27</lastmod>
        <priority>0.8</priority>
    </url>

    <url>
        <loc>https://errors.haskell.org/messages/GHC-00158/index.html</loc>
        <changefreq>weekly</changefreq>
        <lastmod>2023-06-10</lastmod>
        <priority>0.8</priority>
    </url>

    SNIP...
</urlset>

Currently only the html pages for individual errors are indexed.

[BinderDavid]

See also https://www.sitemaps.org/protocol.html#informing for next steps.

[MangoIV]

looks good to me. I have not validated the sitemap though. perhaps we can do this somehow?