docs: add usage, trademark and security disclaimer

[opensource-SantanderAI]

Adds a standard Disclaimer section covering:

• "as is" provision under the project license; not an official Banco Santander product or service; no production support commitment.
• Trademark notice: the license grants no rights over the Santander name/logo (reinforces Apache-2.0 §6 for non-lawyer readers).
• Security: route vulnerabilities through the org security policy instead of public issues.

Org-wide rollout (12 product repos + org profile). Part of the security controls plan (P4).