Clarify namespace model for LanceDB users; add concepts + SDK usage docs

[prrao87]

Fixes #141

Summary

This PR resolves user confusion per #141 by splitting namespace documentation into:

• a concepts/explanation page: what namespaces are in LanceDB, where they fit, and why they matter
• usage/how-to pages (how namespace parameters are used across SDKs).

It also removes the low-value “Feature catalog” page which was noisy (and likely out-of-date) and replaces it with something that's more directly useful for LanceDB users who may have heard of Lance, but don't clearly know how they fit together.

Why do we need this?

Users are seeing namespace in the SDK API reference (for example drop_table) without clear context on the following:

• LanceDB as a catalog-level abstraction, and
• how namespace paths relate to table resolution.

This PR makes that model explicit and easier to navigate as a new user learning how to use LanceDB and manage tables.

Validation

• Regenerated snippets:
  • make snippets
• Verified Python snippet source syntax:
  • uv run pytest tests/py/test_connection.py