RE: Improve docs syntax checking and enable it in the meson build

Dear Nazir,

I created a similar patch with yours because I did not recognize this thread.
Not sure you are still interested in, but here are my comments based on my
experience.

00.
Needs rebase.

01.
I found that postgres core was built when I ran `meson test --suite doc`
command, but ideally it's not needed. Per my experiment, we must clarify
dependencies for the test, something like below.

```
--- a/doc/src/sgml/meson.build
+++ b/doc/src/sgml/meson.build
@@ -337,7 +337,8 @@ test(
'doc' / 'sgml_syntax_check',
meson_bin,
args: ['compile', postgres_full_xml_target],
- suite: 'doc'
+ suite: 'doc',
+ depends: doc_generated,
)
```

02.
Missing update of copyright.

03.
I found that for the autoconf build system, syntax checking cannot work again
if nothing is changed. This behavior is different from the HEAD. Per my
experiment, below can fix the issue.

```
--- a/doc/src/sgml/Makefile
+++ b/doc/src/sgml/Makefile
@@ -201,6 +201,7 @@ MAKEINFO = makeinfo
##

# Quick syntax check without style processing
+.PHONY: postgres-full.xml
check: postgres-full.xml
```

04.
`make check` is slightly slower than HEAD. I think it is OK but others can also
confirm and can test on their env.

HEAD:
```
$ time make check
...

real 0m0.592s
user 0m0.507s
sys 0m0.084s
```

patched:
```
$ time make check
...

real 0m1.217s
user 0m0.973s
sys 0m0.237s
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED