[Docs]: diagrams rendering unreadable in dark mode

[lgecse]

Issue Description

https://docs.agntcy.org/messaging/slim-core/#message-buffer

Some diagrams on this page have elements with bright background that are not looking readable in dark mode. Example:

Prposed Solution

Remove background from these diagrams.

Affected Component

SLIM

Checklist

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

[lgecse]

Closing as this was resolved with content updates. The diagram as shown in the ticket got removed in #163 .