Add runnable code snippets to CTable.where() and sort_by() docstrings

The docstrings for CTable.where() and CTable.sort_by() describe the API well but contain no runnable Examples: section. New contributors exploring the API in a REPL or reading rendered API docs get no concrete illustration of the lazy-view behaviour introduced in this release.

Suggested work: Add an Examples section to both docstrings (NumPy docstring format) showing:
- where(): filter with a simple predicate, with columns=, and with a compound condition
- sort_by(): sort a plain table, sort a filtered view (demonstrating that it stays lazy until iterated), and multi-key sort
- Both should include print() calls that make the lazy-vs-eager distinction visible