Docs: add provider sandbox account checklist template

[sridharkalaibala]

Parent: #34

Problem

Many connector PRs will need a sandbox account, but public issues must not ask people to paste secrets. A reusable checklist helps keep this clean.

Acceptance criteria

• Add a provider sandbox checklist template for docs/research issues.
• Include fields for official docs link, sandbox availability, test data limits, webhook support, and credential storage warnings.
• Explicitly forbid posting API keys, tokens, customer data, or production logs.
• Link the template from contributor integration docs where appropriate.

Out of scope

• Creating real provider accounts.
• Storing credentials in the repo.