Reorganize the documentation so that functions have a logical order

[rcap107]

Describe the issue linked to the documentation

The current ordering of the functions in the documentation does not make much sense:

• DatetimeEncoder
• ToCategorical
• ToDatetime
• StringEncoder
• to_datetime
• TextEncoder
• TableVectorizer

It would be better to instead group objects by function, for example (all Joiners together, all Encoders together etc.). At the moment, there aren't so many functions for this to become an issue, but it may be a problem down the line.

Suggest a potential alternative/fix

No response