feat: support additional analytics providers beyond Google Analytics

[rohilsurana]

Problem

The analytics config schema only supports Google Analytics:

```yaml
analytics:
googleAnalytics:
measurementId: G-XXXXXXXXXX
```

Adding Plausible, PostHog, Segment, or other providers requires a schema rewrite and new provider code.

Suggested Approach

• Extend the analytics schema to support a provider pattern:
  ```yaml
  analytics:
  enabled: true
  provider: plausible # or google, posthog, segment
  config:
  domain: docs.example.com
  ```
• Or support multiple concurrent providers (GA + Plausible)
• The existing `analytics` library already supports plugins — leverage that