[LaTeX] Re-style all admonitions to default to having icon + title

- The colours, and also most icons, are emulated from the recent
  sphinx13.css update (PR #12486, PR #12439).

- Add iconpackage key to sphinxsetup key of latex_elements
  fontawesome5 is used if available, else fall-back to fontawesome.
  If none is available, drop icons and shift titles left.
  In all cases, div.<type>_title-icon keys allow user to set
  arbitrary code to be used.

- Defaults for padding and border-widths of admonitions have been
  changed, now that they all acquire a default background colour.
  The defaults always have horizontal padding plus border-width
  add up to 12.5pt, so contents of all types align exactly vertically.
  Or course user can specify arbitrary values.

- The code inserting the coloured title-row incorporates a work-around
  to a feature of TeX vertical lists when they start with for example
  a colour special.  It would not be necessary with LaTeX of 2023-06-01
  and newer. cf Improve spacing at top of minipages in
  https://www.latex-project.org/news/latex2e-news/ltnews37.pdf

CHANGES.rst

@@ -4,12 +4,24 @@ Release 7.4.0 (in development)
 Dependencies
 ------------
 
+* LaTeX: the ``xcolor`` package is now required.  It is anyhow part of
+  Ubuntu ``texlive-latex-recommended`` which has always been required.
+
+* LaTeX: the ``fontawesome5`` LaTeX package is needed for the default choices
+  of icons now used in admonition titles in PDF output; but if unavailable the
+  PDF build will simply silently omit rendering such icons.  Check the
+  documentation of the ``iconpackage`` key of :ref:`'sphinxsetup'
+  <latexsphinxsetup>` for more.
+
 Incompatible changes
 --------------------
 
 Deprecated
 ----------
 
+* LaTeX: the ``sphinxlightbox`` environment is not used anymore, all types
+  of admonitions use (by default) only ``sphinxheavybox``.
+
 Features added
 --------------
 

bindep.txt

@@ -14,10 +14,13 @@ texlive-ctablestack [platform:rpm]
 texlive-gnu-freefont [platform:rpm]
 texlive-tex-gyre [platform:rpm]
 texlive-pict2e [platform:rpm]
+texlive-fontawesome5 [platform:rpm]
+texlive-xcolor [platform:rpm]
 latexmk [platform:rpm]
 
 texlive-latex-recommended [platform:dpkg]
 texlive-fonts-recommended [platform:dpkg]
+texlive-fonts-extra [platform:dpkg]
 tex-gyre [platform:dpkg]
 texlive-latex-extra [platform:dpkg]
 texlive-luatex [platform:dpkg]