Add sitemap generator

[mroderick]

This pull request is a solution for #2347.

It adds the sitemap_geneator gem to pave the way for a future improvement, where we actually add links like workshops and events to the sitemap, regenerate the file in production and store it in S3, so it survives between deployments.

For now, this will make all the 404 Not found for sitemap.xml go away.

If we want to update the sitemap file, we need to run the rake task.

See https://github.com/kjvarga/sitemap_generator?tab=readme-ov-file#rake-tasks for details on rake tasks.

[olleolleolle]

This adds the structure, we can introduce content as we go.

[olleolleolle]

(Future details here:)

And, in the future we can set up the SitemapGenerator::Sitemap.adapter = SitemapGenerator::AwsSdkAdapter.new(ENV.fetch("SITEMAP_S3_BUCKET")). When we do, remember to update the Sitemap line in public/robots.txt when changing it: SitemapGenerator::Sitemap.sitemaps_host = ENV.fetch("SITEMAP_HOST")

And, the AWS SDK's docs has notes on the two ENV vars to use for credentials; https://docs.aws.amazon.com/sdk-for-ruby/v2/api/Aws/S3/Client.html#initialize-instance_method