feat: Group experimentation commands under experimentation

[developerkunal]

🔧 Changes

Groups the experimentation commands under a single experimentation parent command, mirroring the Management API's experimentation grouping.

Command grouping

• Adds auth0 experimentation (alias exp) as the parent for experiments, feature-flags, and segments. Variations remain nested under feature-flags (auth0 experimentation feature-flags variations ...), matching the API where variations belong to a feature flag.
• Removes the three top-level commands (auth0 experiments, auth0 feature-flags, auth0 segments) in favour of the grouped paths.

Allocation flow (experiments)

• Interactive create/update now presents a variation multi-select (choose 1 to 20) with paging for feature flags that have many variations.
• Enforces the 20-allocation maximum and the whole-number 1–100 weight bounds on the non-interactive --allocations flag, so the flag and interactive picker validate identically.

Update behaviour

• update reads the current resource first, pre-fills interactive prompts with the existing values, and sends only the fields that actually changed. Allocations are PUT-like and replaced wholesale when edited.
• Interactive update opens editors pre-filled with current values for feature-flag parameters, variation overrides, and segment rules, matching the create experience. It also offers to edit the experiment's assignment config and allocations behind a confirmation, including the percentage/segment strategy selection.
• The update output now renders the full API response (feature flag, auth flow, allocation strategy, allocations, created, etc.) instead of only a subset of fields.

📚 References

Follow-up to #1562.

🔬 Testing

• Unit tests updated across experiments, feature-flags, and segments update commands to mock the new Get call and assert diff behaviour, full-response rendering, and allocation validation (empty, over-limit, out-of-range weight).
• Added a segments update test suite (previously none).
• Integration test cases and helper scripts updated to the new experimentation command paths, plus new cases covering allocation validation on both create and update.
• Docs regenerated via make docs.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)