[Docs]: Update descriptions of CSIT about "Agent Gateway Protocol"

[XiaoTianCan]

Issue Description

The term "Agent Gateway Protocol" apears in https://docs.agntcy.org/csit/csit/, but it seems not appear anywhere else. May need to change it to SLIM?

Prposed Solution

Update the description.

Affected Component

CSIT

Checklist

• I have read the contributing guidelines.
• I have verified this does not duplicate an existing issue.