docs: add pricing info to embeddinggemma-300m

[Tmalone1250]

Summary

Fixes #27490

This PR updates the embeddinggemma-300m.json model definition to include missing pricing information.

Changes:

• Added the properties block containing pricing metadata.
• Set the price to 0 (Free) to align with current Workers AI pricing for this model.
• Matched the schema structure used in qwen3-embedding-0.6b for consistency.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry?

• (Note: I have left this unchecked as this is a metadata fix for an existing model page, not a new feature launch, but I am happy to add one if requested.)

• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

Contribution by Gittensor, see my contribution statistics at https://gittensor.io/miners/details?githubId=180482232