feat: Refactors the 3d-accessibility-features sample.

[willum070]

This PR does the following things:

• Replaces the previous programmatic map creation with the declarative <gmp-map-3d> element in index.html, setting map options directly via attributes and removing the old #map div.
• Refactors index.ts to select and use the gmp-map-3d element from the DOM, instead of instantiating Map3DElement in code.
• Changes library imports to use Promise.all for parallel loading of required libraries, improving efficiency and clarity.
• Updates copyright years to 2026 in all files.