docs(multi-provider): Document Track Method Support in Multi-Provider

[jonathannorris]

This PR

This PR updates the Multi-Provider documentation in the OpenFeature specification to document the new track method support that was added in PR #1323.

📚 New Documentation Sections

• Track Method Support: Added a section explaining how the Multi-Provider implements tracking functionality
• Introduction Section: Updated use case examples to highlight tracking capabilities
• BaseEvaluationStrategy: Added shouldTrackWithThisProvider method to the abstract class

Code Examples Added

// Basic tracking usage
const multiProvider = new MultiProvider([
  { provider: new ProviderA() },
  { provider: new ProviderB() }
])

await OpenFeature.setProviderAndWait(multiProvider)
const client = OpenFeature.getClient()

// Track events across all ready providers
client.track('purchase', { targetingKey: 'user123' }, { value: 99.99, currency: 'USD' })

[chrfwow]

Do we also want STALE providers to receive tracking calls?