docs: finalize space/folder CLI documentation after Space Plugin rollout

[sandhyasun]

Follow-up to #13 / PR #24.

The current README was updated with factual changes only. The following items are intentionally deferred until rollout timing is clearer:

• Revisit quick-start examples: replace dremio folder create "Analytics" with dremio space create once Space Plugin is the default
• Update API mapping table to include space.* endpoints
• Add a migration note for users moving from folder create to space create for top-level spaces
• Remove transitional compatibility language once pre-SP cluster support is no longer needed