This only affects 5.1 branch.
While releasing 5.1.13.Final, I got a failure when building the integration guide:
rendering Book(integrationsGuide) en-US/html
Extending script classloader with the jdocbookXsl dependencies
redirecting console output to file /home/gbadner/git/hibernate-orm-5.1.13/documentation/target/docbook/work/integrationsGuide/log/console-en-US-html.log
Error on line 1 column 1 of http://docbook.org/xml/5.0/dtd/docbook.dtd:
Error reported by XML parser: The markup declarations contained or pointed to by the document type declaration must be well-formed.
Resetting console output
This can be fixed by changing "http://docbook.org/xml/5.1/dtd/docbook.dtd" to "https://www.oasis-open.org/docbook/xml/5.0b5/dtd/docbook.dtd"
Fixed in 5.1 branch. This bug does not affect other branches.