Re: documentation structure

1. manually change line 4 in split_func_sgml.py and run the script.
2. git apply v6-0001-all-filelist-for-directory-doc-src-sgml-func.patch
now you can "ninja doc/src/sgml/html"
The logic is simple, but I am using verbose variable names, that's why
split_func_sgml.py size is large.

adding a new directory: doc/src/sgml/func
new directory files: doc/src/sgml/func/allfiles.sgml and others.
For filenames, we can use "func-pattern" or just "pattern". right now,
I use the prefix "func".
each newly created file validates by: only 2 "sect1" exists, each file
has only "<sect1 id=" pattern.

the following ar content of doc/src/sgml/func/allfiles.sgml

<!--
doc/src/sgml/func/allfiles.sgml
PostgreSQL documentation
Complete list of usable sgml source files in this directory.
-->

<!-- function references -->

<!ENTITY func SYSTEM "func.sgml">
<!ENTITY func-logical SYSTEM "func-logical.sgml">
<!ENTITY func-comparison SYSTEM "func-comparison.sgml">
<!ENTITY func-math SYSTEM "func-math.sgml">
<!ENTITY func-string SYSTEM "func-string.sgml">
<!ENTITY func-binarystring SYSTEM "func-binarystring.sgml">
<!ENTITY func-bitstring SYSTEM "func-bitstring.sgml">
<!ENTITY func-matching SYSTEM "func-matching.sgml">
<!ENTITY func-formatting SYSTEM "func-formatting.sgml">
<!ENTITY func-datetime SYSTEM "func-datetime.sgml">
<!ENTITY func-enum SYSTEM "func-enum.sgml">
<!ENTITY func-geometry SYSTEM "func-geometry.sgml">
<!ENTITY func-net SYSTEM "func-net.sgml">
<!ENTITY func-textsearch SYSTEM "func-textsearch.sgml">
<!ENTITY func-uuid SYSTEM "func-uuid.sgml">
<!ENTITY func-xml SYSTEM "func-xml.sgml">
<!ENTITY func-json SYSTEM "func-json.sgml">
<!ENTITY func-sequence SYSTEM "func-sequence.sgml">
<!ENTITY func-conditional SYSTEM "func-conditional.sgml">
<!ENTITY func-array SYSTEM "func-array.sgml">
<!ENTITY func-range SYSTEM "func-range.sgml">
<!ENTITY func-aggregate SYSTEM "func-aggregate.sgml">
<!ENTITY func-window SYSTEM "func-window.sgml">
<!ENTITY func-merge-support SYSTEM "func-merge-support.sgml">
<!ENTITY func-subquery SYSTEM "func-subquery.sgml">
<!ENTITY func-comparisons SYSTEM "func-comparisons.sgml">
<!ENTITY func-srf SYSTEM "func-srf.sgml">
<!ENTITY func-info SYSTEM "func-info.sgml">
<!ENTITY func-admin SYSTEM "func-admin.sgml">
<!ENTITY func-trigger SYSTEM "func-trigger.sgml">
<!ENTITY func-event-triggers SYSTEM "func-event-triggers.sgml">
<!ENTITY func-statistics SYSTEM "func-statistics.sgml">