Voice memory wip

[Pash10g]

This pull request updates the README.md for the voice-memory-demo app to clarify setup instructions and improve accuracy regarding search indexes and article references.

Documentation improvements:

• Updated the article link to point to the published Dev.to article instead of a relative path.
• Clarified that the VOYAGE_AI_API_KEY is not strictly optional in the environment variable example.

Atlas Search index instructions:

• Clarified that the code creates the required Atlas Search indexes on startup, and marked manual creation steps as optional for both vector and text indexes. [1] [2]