docs: build the same HTML page set for every language

[grandixximo]

Problem

include::d partials (hal/halscope, hal/halshow, help/rtfaults, help/tklinuxcnc) have no standalone English page: their content renders inside the chapter that includes them. The translated build, though, rendered them as standalone pages (de/hal/halscope.html and so on), because the per-language HTML target list is derived from po4a.cfg, which lists partials as translation units.

Each such translated page then carried a language switcher linking to en/hal/halscope.html, which does not exist. lang_switcher_postprocess.py greyed the entry but kept the dead href, so it still 404s. This is the bulk of the broken cross-language links an HTML link checker reports in the translated trees.

Fix

Restrict the per-language HTML targets to the same curated page set English builds (DOC_HTML_PAGE_STEMS, from DOC_SRCS_EN). Partials are no longer rendered standalone in any language; their text is still translated and inlined into the translated chapters, and xref_resolver.rb already redirects partial anchors to the including page.

Verification

Full 7-language doc build (de es fr nb ru uk zh_CN):

• Every language now builds the same 132 pages (de was 136 before, the 4 extras being the partials).
• The 4 partials no longer exist as standalone pages in any language.
• No remaining links to those en/... pages anywhere in the output.
• Partial content still inlined in the translated chapters (e.g. halscope in de/hal/tutorial.html).
• A second build is a no-op.

[hdiethelm]

@grandixximo Might be rebase to master, so the links are checked?

[grandixximo]

Fixes 28 links, other will probably have to be tackled separately, sid is being flaky.

[hdiethelm]

Nice, only fragments left and one link:

Processing	file:///home/runner/work/linuxcnc/linuxcnc/docs/build/html/es/plasma/plasma-cnc-primer.html


List of broken links and other issues:
http:./qtplasmac.html	
  Line: 281
  Code: 500 No Host option provided
 To do: This is a server side problem. Check the URI.

The cross links are indeed broken:

Processing	file:///home/runner/work/linuxcnc/linuxcnc/docs/build/html/de/getting-started/updating-linuxcnc.html


List of broken links and other issues:
file:///home/runner/work/linuxcnc/linuxcnc/docs/build/html/de/getting-started/updating-linuxcnc.html	
 Lines: 33, 75
  Code: 200 (no message)
 To do: Some of the links to this resource point to broken URI fragments
	(such as index.html#fragment).
The following fragments need to be fixed:
	start:update-no-network       	Line: 75

https://linuxcnc.org/docs/devel/html/de/getting-started/updating-linuxcnc.html
Click on Updating without Network.

Line 75: <a href="[#start:update-no-network](view-source:https://linuxcnc.org/docs/devel/html/de/getting-started/updating-linuxcnc.html#start:update-no-network)">
Line 297: <h2 id="getting-started:update-no-network">2. Aktualisieren ohne Netzwerk</h2>

Notice getting-started:update-no-network vs. start:update-no-network.

I guess this should be a separate PR. It's probably also a translation issue, in the English version, it is fine.

[BsAtHome]

sid is being flaky.

Hm, it seems that both SID instances hang on the test Running test: ./tests/motion-logger/basic and having several errors previously. It may be a change in a supplied library?

[hdiethelm]

sid is being flaky.

Hm, it seems that both SID instances hang on the test Running test: ./tests/motion-logger/basic and having several errors previously. It may be a change in a supplied library?

I have a PR canceling to long taking jobs automatically, so no one has to do it manually:
#4207

It might be a temporary issue in sid or also a real library change. This can be investigated in a docker or VM. I would just wait a few days to see if it fixes itself... ;-)