Third-party manual, doc-base side

[alfsb]

This is the doc-base side of php/doc-en#5682 , for the splitting of third-party manuals from the main manual, with the minimal conventions possible. I took the path of also minimizing any XML output differences. In fact, the only output change occurs in the DOCTYPE node, the XML is identical. Also, the code is already prepared for any disappearance of actual DTD entity files.

As this is a big change, I plan to leave this open for almost two weeks.

The planned merge order is doc-en 5682 on July 30, and this in July 31. In the meantime, preparing for the actual disappearance of DTD entity files.

[alfsb]

Comments and reviews wellcome.

Build breakage is expected, as manual.xml still does not exist on doc-en.

[alfsb]

Tomorrow is the big day. With this PR, doc-base becores almost independent of main manual manual.xml, so extension and external manual also becomes independent. Some last comments, before the big push merge?

[alfsb]

Merged. No differences detected on XML body so far. I will do some more tests, on all languages, to be sure. Next in line, DOCTYPE minimization, XML trim and possibly adding a warning on GH CI builds about invalid results.