XML Entities, individual and grouped

[alfsb]

This PR creates a new doc-base/scripts/entities.ent file, that is called from configure.php but can also be called from the command line.

The new script start looking global.ent, manual.ent and remove.ent in each doc-lang repository. Besides the .ent extension, these are normal. XML files, that uses the same namespaces as manual, so small entities placed here can be namespace clean(er),

The new script also starts looking for an entities/ dir in each doc-lang repository, and loads any .xml file found here as an individual entity file, so bigger entities get easier to edit and can be now revchecked individually.

Included are two other scripts, dtdent-conv.php and dtdent-split.php, that bulk convert (or split) big files of DTD Entities into XML Entities. These tools are not necessary for implantation.

This will make entity experimentation a lot easier, and is the enabling step into splitting language-entities.ent file. This works well, but is another possibly big change, so I do not plan to push for this until 2025, or the PHP 8.4 doc changes slow down, or if there is some demand for early experimentation.