Idempotent build: manual.xml.in

[alfsb]

This PR removes the last non-PhD related, non idempotency of manual. All other are PhD related and will need coordination.

Tested on all languages, with and without CHM enabled. Few languages have broken XML files inside chmonly/. This PR need to be tested on Windows, in particular, by builders of CHM manuals, to see if nothing got broken in this process. The tests showed identical XML output, but changes on DTD area.

Opened to review, but I'm not expecting merging this before 2025. There is nothing blocking this, really, only to avoid injecting possible breakages near new year festivities.

The broken chmonly/ files I found are:

• fr/chmonly/search.xml
• ja/chmonly/usingchm.xml

[Girgias]

Wouldn't it be better to use process instruction for the inclusion of the translation layer and CHM?

[alfsb]

The PI alternative for this would be at XML parse, or as after parse fixup?

[alfsb]

Wouldn't it be better to use process instruction for the inclusion of the translation layer and CHM?

I do not think that processing instructions will work, for the same reason that XInclude wont work in general: entities. Files at chmonly/ have entity references. And entity declarations do not survive parse time, nor libxml appear to share entity declarations between files (this last bit is something I will try more experiments latter on).

This PR only focuses on making manual building as idempotent as possible, and for this, it's only necessary to avoid writing/changing files outside temp/.