Fix canonical base to /docs and retire the single-page probe

- seo.metatags.canonical -> https://www.checklyhq.com/docs so Mintlify's
  auto-generated canonicals keep the /docs mount (bare origin dropped it).
  Supersedes #421.
- Remove the temporary trailing-slash canonical from cli/overview.mdx so
  it rejoins the control group.

This makes every non-experiment page emit www + /docs + no-slash — the
control baseline for the trailing-slash canonical experiment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Dgiordano33

cli/overview.mdx

@@ -2,7 +2,6 @@
 title: Checkly CLI
 description: 'Code, test, and deploy synthetic monitoring at scale with the Checkly CLI.'
 sidebarTitle: 'Overview'
-canonical: 'https://www.checklyhq.com/docs/cli/overview/'
 ---
 
 import MainCicdCards from "/snippets/main-cicd-cards.mdx"

docs.json

@@ -11,7 +11,7 @@
     "title": "Getting started with Checkly - Checkly Docs",
     "description": "The Application Reliability Platform built for modern engineering teams.",
     "metatags": {
-      "canonical": "https://www.checklyhq.com"
+      "canonical": "https://www.checklyhq.com/docs"
     }
   },
   "favicon": "/racoon_favicon.png",