Close #13597 (LaTeX table in merged cell of parent table)

[jfbu]

The position of the nested table will not be optimal, but it seems that generally speaking contents of multicolumn cells are rendered aligned left in general anyhow.

Purpose

Fix #13597

The produced LaTeX mark-up was completely mangled. With this patch, LaTeX mark-up compiles to PDF.

TODO: customize placement of contents (here a table) if in a multi-column cell. It seems alignment is always left (l type column) and can't be customized.

I have tested this on my stock of table tests for PDF rendering and no changes appear to be induced. So I will merge after testing completes.

sorry for multiple forcepush, but I am on a new system and I had not yet installed the testing infrastructure at my locale and mypy has tortured me

[jfbu]

I had to relaunch the CI for Windows on this PR and again after merge due to failed linkcheck test related to a time-out situation

see https://github.com/jfbu/sphinx/actions/runs/15508817453/job/43667012833#step:9:4648

FAILED tests/test_builders/test_build_linkcheck.py::test_anchors_ignored - assert not "index.rst:2: [timeout] http://localhost:57370/#top: HTTPConnectionPool(host='localhost', port=57370): Read timed out. (read timeout=0.25)\nindex.rst:1: [timeout] http://localhost:57370/#!bar: HTTPConnectionPool(host='localhost', port=57370): Read timed out. (read timeout=0.25)\n"

cc @AA-Turner