docs: add SignalR hub API security evidence gates

[mz13stars-cpu]

Fixes #1187.

Summary

• Adds a SignalR hub security section to the .NET API security supplement.
• Covers hub inventory, browser credential/origin evidence, method/group authorization, query-string access-token handling, and buffer/detailed-error controls.
• Adds grep patterns and Microsoft SignalR security/auth references.

Verification

• git diff --check
• Markdown code fence balance check
• Required marker checks for MapHub, hub authorization, Groups.AddToGroupAsync, access_token, buffer limits, and SignalR references.

Bounty

Requesting consideration under the SecuritySkills bounty program as an Improver contribution. Preferred payment method can be provided privately after acceptance.