Doc: add check to detect non-breaking spaces in the docs.

Doc: add check to detect non-breaking spaces in the docs.

There were multiple instances where accidentally adding non-breaking
space (nbsp, U+00A0, 0xc2a0 in UTF-8) to sgml files. This commit adds
additional checking to detect nbsp. You can check the nbsp by:

make -C doc/src/sgml check

or

make -C doc/src/sgml check-nbsp

Authors: Yugo Nagata, Daniel Gustafsson
Reviewed-by: Tatsuo Ishii, Daniel Gustafsson
Discussion: https://postgr.es/m/20240930.153404.202479334310259810.ishii%40postgresql.org