docs: conceptual documentation for key concepts

[lostmygithubaccount]

This issue is not fully thought through, opening for public discussion/input.

Ibis has some number of concepts that should be captured in the conceptual documentation -- the "nouns" that are used throughout the rest of the documentation. This may include:

• tables and views
• expressions (table and column?)
• type system
• UDFs
• connections (backends)
• batch vs streaming

This is not an exhaustive list.

We should also document important concepts like the architecture and principles for developing Ibis.