docs: update README for dremio space command group

[sandhyasun]

Follow-up to #13 / PR #24.

The README was intentionally left out of the space command PR to keep the diff focused. The following sections are now stale and need updating:

1. Command summary table (line 118) — add dremio space row (list, get, create, delete)
2. Quick-start examples (lines 141–143) — replace dremio folder create "Analytics" with dremio space create "Analytics"; keep dremio folder create Analytics.reports for the nested folder example
3. CRUD table (line 217) — split Spaces/Folders row so spaces map to space list/get/create/delete and folders map to folder list/get/create/delete
4. folder create SQL description (line 229) — currently says "folder create uses CREATE SPACE for top-level" which is no longer true; update to reflect deprecation warning + server-driven behavior
5. API mapping table (lines 255–257) — add space list, space get, space create, space delete alongside the existing folder entries
6. File tree (line 363) — add space.py entry

[sandhyasun]

Closing — README updates for dremio space belong in PR #24 directly.