doc: improve build for non-Latin1 characters

Bruce Momjian <bruce@momjian.us>

Commit: 641a5b7a1447954076728f259342c2f9201bb0b5
Author: Bruce Momjian <bruce@momjian.us>
Date: 2024-11-01T16:46:51Z
Releases: 18.0
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

Files

Documentation touched