Improve developer experience with Kotlin DataFrame documentation: new structure and user guides

[zaleslaw]

• Probably it's worth to repeat at the bottom the whole structure to be discoverable https://ktor.io/docs/welcome.html
• Make working with Databases easier with copy-paste https://www.jetbrains.com/help/exposed/working-with-database.html#connecting-to-a-database

[Jolanrensen]

I would even go so far to put "Integrations" on the top-level of the documentation website. It could have pages dedicated to dataframe-excel, dataframe-arrow, and also how to integrate unsupported data sources. Currently this information is nested down 3 levels in https://kotlin.github.io/dataframe/read.html

[koperagen]

Create a page with all functions in alphabetic order. Each function should have a header, link to page, a description and meta information, like opposite operation: for group it will be ungroup, related operations, tags like "Access", "Modify"
All this information is now encoded in operation tree, making it very hard to find a function there by the name
We should have this page and then flatten operations

[Jolanrensen]

From #1253:

1. Library Overview page (https://kotlin.github.io/dataframe/overview.html) hadn't been changed since it was a welcome page. Its purpose is not very clear right now. I think we should rewrite it.
2. Get started with Kotlin DataFrame (https://kotlin.github.io/dataframe/gettingstarted.html) clashes with Quickstart Guide (https://kotlin.github.io/dataframe/quickstart.html) and can confuse users - where should I actually start? Maybe rename Get started with "setup"? Not sure.
3. https://kotlin.github.io/dataframe/extensionpropertiesapi.html is very important, especially for beginners but it's deeply hidden (3rd level!).
4. Not sure if datamodels/nan-na/number unification should be on the 1st level