feat: add initial iconography guidelines and starter set

[hriszc]

Fixes #119

This is a first structured pass at a unified iconography system for AsyncAPI:

• add a dedicated iconography guideline page under the brand docs
• define shared grid, stroke, color, and contribution rules
• seed the system with four reusable SVG icons for core platform concepts: event, channel, broker, and schema
• link the new section from the main brand guideline index

This keeps the first PR intentionally small so the visual language can be reviewed before expanding the full icon library.