* add dovecot-fts-xapian
update Docker to build from debian bullseye slim, as it contains
packages for fts-xapian.
update Docker to install dovecot-fts-xapian.
update docs with instructions on how to enable fts-xapian or fts-solr
and what considerations to take into when deciding.
* address review feedback
* update backport method to previously proposed approach (which was lost in a forced push)
Also removed the FAQ inline heading link for Rancher. It's not a relevant link (as the question already expects knowledge of Rancher), and breaks out of the bg colour heading style due to the HTML generation logic from mkdocs.
