pgsql: doc: Documentation updates for online checksums

doc: Documentation updates for online checksums

A set of documentation updates for the online checksums work, found via
manual as well as LLM-guided review.

* The state diagram was re-done to be improve readability and be
more in line with the look and feel of other diagrams.
* Paragraph about mismatched states moved to its own sect2.
* The documentation for the functions to enable/ disable checksums
failed to mention that they are superuser only.
* The documentation for the checksums progress reporting had
accidentally omitted the work "fork" and for the blocks_* columns.
The values are per fork but the documentation made it seem they
were per relation.
* Add the state image to the meson buildfile

Reported-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://postgr.es/m/5b08b4d1-0982-4b77-bac5-3bdffc6583f5@iki.fi
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2bd7519a8150e7607c7ea8a89067bd0888526ddb

Modified Files
--------------
doc/src/sgml/func/func-admin.sgml | 13 ++-
doc/src/sgml/images/datachecksums.gv | 61 ++++++++++---
doc/src/sgml/images/datachecksums.svg | 163 ++++++++++++++++++++++------------
doc/src/sgml/images/meson.build | 1 +
doc/src/sgml/monitoring.sgml | 4 +-
doc/src/sgml/ref/pg_checksums.sgml | 2 +-
doc/src/sgml/wal.sgml | 62 +++++++------
7 files changed, 204 insertions(+), 102 deletions(-)