[NA] [DOCS] fix: point changelog canonical URLs at the changelog index

The 2026-07-13 and 2026-07-20 entries set canonical-url to a per-date path
(/docs/opik/changelog/<date>), but Fern renders changelog entries into the
changelog section rather than standalone per-date pages, so both URLs 404. The
other 59 entries all point at /docs/opik/changelog; match them.

This is why "Docs - Preview link" has been failing the broken-link check on
every docs-touching PR.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: jverre

apps/opik-documentation/documentation/fern/docs/changelog/2026-07-13.mdx

@@ -1,5 +1,5 @@
 ---
-canonical-url: https://www.comet.com/docs/opik/changelog/2026-07-13
+canonical-url: https://www.comet.com/docs/opik/changelog
 ---
 
 ## Per-Evaluation Spend Budget for LLM-as-Judge Evaluators

apps/opik-documentation/documentation/fern/docs/changelog/2026-07-20.mdx

@@ -1,5 +1,5 @@
 ---
-canonical-url: https://www.comet.com/docs/opik/changelog/2026-07-20
+canonical-url: https://www.comet.com/docs/opik/changelog
 ---
 
 ## Redesigned Optimization Run Overview