Recent change to site broke links and presentation

The page at https://flagon.apache.org/ has developed lots of issues.

Various broken links, and the site does not render properly.

It looks like the page generation is broken, as links are appearing as:

href="{{ '/userale' | prepend: site.baseurl }}"

(for example)

Open https://flagon.apache.org/ and inspect the page-generation source responsible for the reported template expression. Reproduce the broken links and rendering, then verify the generated pages no longer expose template syntax and that links and presentation work across the site.

Written by the indexing model from the issue text.