Clarify greptimedb's supported functions

[MichaelScofield]

Now the "Functions" page is a little too fuzzy. It's hard for user to browser what functions greptimedb support. At lease when I want to find the UDAFs, I have to go with the source code. So I think a total refactor with function page is necessary that:

• separate greptimedb's UDF/UDAF from datafusion's, list them explicitly
• we can still link to datafusion's site for its supported functions
• maybe better use tables for displaying all the functions? Tables are compact, good for eyes.
• write examples for all our own functions
• create a sub-page for "python script as function", and "use function in python script"
• write the doc for "how to write a UDF/UDAF in greptimedb" as a sub page, or put it elsewhere then link it