Proof-of-concept for reading time

[tomschr]

As discussed in our doc tools meeting on Jan 9th 2025, reading time should be calculated with XSLT (and not with JS due to performance issues during loading).

In the root directory, call

$ xsltproc contrib/reading-time/reading-time.xsl contrib/reading-time/sample-1.xml
<?xml version="1.0"?>
<result xmlns:d="http://docbook.org/ns/docbook">
  <word-count>22</word-count>
  <reading-time unit="min">1</reading-time>
</result>