Add a UserGuide for type conversion with `convertTo`

[zaleslaw]

Again, the discussion here #998 (comment) contain very important ideas and hints for the experienced user

We should summarize all our recommendations into one Guide, which help a user to convert/handle types properly.
Especially, will be great if some copy-paste templates will be there

[Jolanrensen]

Note #1420 improves on convertTo docs on the website as well already

[Jolanrensen]

@zaleslaw now #1420 is merged: https://kotlin.github.io/dataframe/convertto.html
What else would you like to see in the documentation for convertTo<> {}?