Automate documented functions

In the Pinot docs[1], only a subset of available functions are documented. I think it would be highly valuable to automate listing any function found in code[2] to be part of those docs. Even if the default behaviour was to link to the definition in code, it would still provide better discovery for anyone writing queries and only looking at the docs.

[1] https://docs.pinot.apache.org/configuration-reference/functions
[2] https://github.com/apache/pinot/tree/master/pinot-core/src/main/java/org/apache/pinot/core/operator/transform/function

Start with the Pinot functions documentation at docs.pinot.apache.org/configuration-reference/functions and the Java function definitions under pinot-core/src/main/java/org/apache/pinot/core/operator/transform/function. Determine how functions are currently discovered and documented, then define a generated listing that covers every function found in code. Done means the documentation exposes all discovered functions, including links to definitions where full descriptions are unavailable.

Written by the indexing model from the issue text.