Minimize DTD subsystem

[alfsb]

With this change, the preamble of all manual.xml can be reduced to just:

<?xml version="1.0" encoding="utf-8"?>
<?do-not-translate?>

<!DOCTYPE PHPDOC [
<!ENTITY % configure SYSTEM "./temp/conf.dtd">
%configure;
]>

And cross-repository PRs about the DTD subsystem will disappear. Also allows the removal of the ⟨ entity, the only case of entity in XML attribute on manual. Can be merged as is, but a similar change on doc-en will be done. Expected conflict with the-big-trim branch, that will be merge first.

Comments and reviews are welcome. Plan to merge this in one week.