[google-genai] Add standard definitions to Google GenAI systems

[Rima-ag]

#2053

Changes

Create documentation for the common span definition of Google systems. This will make it easier to review #3486 that is adding Google-specific semconv.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• Links to the prototypes or existing instrumentations (when adding or changing conventions)

[github-actions]

This PR has been labeled as stale due to lack of activity. It will be automatically closed if there is no further activity over the next 7 days.