docs: add Sardis provider integration page

[Efe Baran Durmaz (EfeDurmaz16)]

Summary

• Adds Sardis as a community integration provider
• Sardis enables AI agents to make policy-controlled payments through non-custodial MPC wallets
• Package: sardis-langchain on PyPI

What is Sardis?

Sardis is the Payment OS for the Agent Economy — infrastructure enabling AI agents to make real financial transactions safely with spending policy guardrails.

Tools provided

• SardisPaymentTool — Execute policy-controlled payments
• SardisBalanceTool — Check balance and spending limits
• SardisPolicyCheckTool — Pre-check payment policy compliance

Links: Website | Docs | GitHub

[Efe Baran Durmaz (EfeDurmaz16)]

Closing - submitted to the correct repo (langchain-ai/docs) instead. See langchain-ai/docs#2990