Migrate Overview page to the Concept page and collect all Library's conceptions and design decisions under one section

[zaleslaw]

I'd like idea of Polars docs, where they put some stuff under abstract Concepts naming https://docs.pola.rs/user-guide/concepts/, we could move in the same section our Data Structures, Hierarchichal DataFrames, our tight integrations with Kotlin and IDEA, also copy/immutability concept on each operation, our DSL, plugins approach

Candidates to move

• https://kotlin.github.io/dataframe/types.html + Combine it with https://kotlin.github.io/dataframe/hierarchical.html
• https://kotlin.github.io/dataframe/nanandna.html
• https://kotlin.github.io/dataframe/numberunification.html
• Flatten https://kotlin.github.io/dataframe/apilevels.html (it's only two for now, you could move String API on the API page, and the second could be Extension properties)
• add a page about immutability (not in-place operations)