docs: Add beginner-friendly comments to hello_blockchain Move example

[Sandeep2566]

This PR adds clear comments explaining how the hello_blockchain Move example works.
It improves readability for new Move learners and clarifies resource storage and event usage.
No logic or behavior was changed.

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Note

Adds explanatory comments across hello_blockchain.move to clarify resource storage, event emission, view/entry functions, and the unit test; no behavior changes.

• Docs (Move example):
  • Clarify MessageHolder resource purpose and storage under user accounts.
  • Document MessageChange event and its use during updates.
  • Add inline explanations in get_message (existence check, read-only) and set_message (account addr, create-or-update flow, event emission, write).
  • Comment the test sender_can_set_message to explain setup, call, and assertion steps.

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