Create manual subsections for each function

[lukaseder]

This is a continuation of #10078

The current version of the manual has only one section per function type, e.g. "string functions":
https://www.jooq.org/doc/latest/manual/sql-building/column-expressions/string-functions/

It would be good to have subsections for each type, such that each function can be documented individually, with examples. This is particularly useful for aggregate functions and window functions, where each function has a few specialties worth mentioning.

The relevant sections include:

• Aggregate functions (see also: Create subsections in the manual for aggregate function features #10079)
  • Most aggregate functions
  • Statistical aggregate functions (variance, linear regression. I'd like to find a meaningful explanation that explains what these are for)
• Window functions
• Array functions

We probably also need a new section for operators

• String operators
• Numeric operators
• Etc.?

[lukaseder]

Most functions are documented by now. Will add more in the future, if I discover missing documentation

[lukaseder]

Still no dedicated sections for window functions!