fix: provide absolute URL comments for graphic assets for copy-paste

[willum070]

This PR resolves an issue where developers copying code snippets from the advanced-markers-graphics and 3d-marker-graphics samples would encounter broken images because the local relative assets (./public/... or images/...) are not present in their local environments or JSFiddle.

Changes:

• Added clear, educational comments above image instantiations providing the absolute hosted URL (https://maps-docs-team.web.app/...) for the required assets (beachflag.png, google_logo_g.svg, and 192px.svg).
• Retained the import.meta.url Vite pattern for the actual execution lines.

Why this approach?
This pattern provides the best of both worlds:

1. Fixes copy-paste: Documentation readers are explicitly given the absolute URL they need to make the snippet work immediately in their own projects.
2. Preserves local development: Local Vite development (npm start) continues to resolve the local assets seamlessly, avoiding the "chicken-and-egg" issue of requiring an asset to be deployed to production before it can be previewed locally.
3. Self-contained repository: Ensures the project structure remains self-contained with its own graphic assets.

Affected Samples

• advanced-markers-graphics
• 3d-marker-graphics

Testing

• Verified that local Vite builds and npm start continue to work flawlessly.
• Verified standard Prettier and ESLint rules pass.

Fixes b/499325122

Change-Id: I495fc4a4ef5a6563806250567396e6b28f5a7f14