fix(bridge): important details on DRT

[storopoli]

Description

While testing the Bridge Client, I've realized that we're missing some important details on the Deposit Request Transaction (DRT) about:

1. that the multisig N-of-N is a Schnorr aggregated sig
2. that the OP_RETURN needs to be the first output

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Type of Change

• New Document
• Update to Existing Document
• Bug Fix
• Question/clarification
• Other (please describe):

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Checklist

• I have reviewed the existing documentation to avoid duplication.
• The new or updated document includes clear and concise information.
• All relevant sections (e.g., introduction, usage examples, references)
  are included.
• The document follows the project's style guide and formatting rules.
• I have included any necessary references or external resources.
• Spellcheck and grammar check have been performed.
• (For updates) I have verified that the changes reflect the current state
  of the project.

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Reviewer Checklist

• The purpose and scope of the document are clear.
• The document is easy to understand and follow.
• There are no typos or grammatical errors.
• All necessary sections are included and well-structured.
• The document is consistent with the project's style guide.
• Any referenced links or resources are valid and appropriate.

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Thank you for contributing to our documentation!

[storopoli]

ready merge if approved

[john-light]

LGTM