doc: improve build for non-Latin1 characters
Add README.non-ASCII to explain non-ASCII doc behavior; some text moved
from release.sgml.
Change UTF8 SGML characters to use HTML entities.
Remove unnecessary UTF8 spaces.
Add SVG file check for check-nbsp target.
Add dummy 'pdf' Makefile target.
Reported-by: Yugo Nagata
Discussion: https://postgr.es/m/
20241011114122.
c90f8a871462da36f2e2afeb@sraoss.co.jp
Backpatch-through: master
