Intrinsic functions: generate documentation

[chillleader]

What should we do?

We implemented intrinsic functions in #3843. We need to provide some documentation for the users/connector developers where all parameters of the out-of-the-box intrinsic functions are described.

e.g.:

• base64 - can accept either string or document
• generateLink - can only accept documents

and so on.

This can be generated because we already have reflection access to the params definition. We can do this using a Maven plugin, similarly to element-template-generator-maven-plugin.

Why should we do it?

To make it easier to know which functions can be used out-of-the-box and which parameters they can accept.