docs: add offchain message delivery guide (backport of #24583 to v5-next)

[AztecBot]

Backports #24583 (docs: add offchain message delivery guide, by @ciaranightingale) onto v5-next.

What's included

• New page offchain_message_delivery.md — full offchain delivery workflow (contract emission → sender-side extraction → transport → offchain_receive → processing lifecycle → TXE testing), pulling real code from the token contract and its offchain-delivery test via #include_code.
• note_delivery.md — replaces the stale process_message flow and outdated JS example with the current offchain_receive / offchainMessages aztec.js API, adds a cross-link to the new page, and sentence-cases headings.
• offchain_messages.nr — doc comment updated: process_message → offchain_receive (injected by the #[aztec] macro).
• Test file — adds docs:start/end:token_transfer_offchain_delivery_test markers so the new page can #include_code a real test.

Backport notes

• The original PR's diff applies cleanly on v5-next; the only manual step was re-applying three heading-casing edits in note_delivery.md whose surrounding context differs on this line (v5-next carries an extra "Tagging secret strategy" section). Content is otherwise identical to docs: add offchain message delivery guide #24583.
• Both #include_code targets resolve on v5-next: the transfer_in_private_with_offchain_delivery marker in token_contract/src/main.nr already exists, and the test marker is added by this PR.
• This is an independent PR, not a cherry-pick: docs: add offchain message delivery guide #24583 is still open/unmerged on next, so the two release lines carry the same docs change separately.
• Base is v5-next (not the default branch), so no auto-close keywords are used.

Refs #24583.

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