[MOAR] Post hog toolkit docs (#537)

* Add PostHog API authentication documentation

- Add Authentication section with step-by-step instructions for obtaining Personal API Key
- Explain how to determine the correct server URL for PostHog Cloud and self-hosted instances
- Emphasize that POSTHOG_SERVER_URL must include the https:// protocol
- Update all tool secrets descriptions throughout the page to:
  - Link directly to PostHog account settings
  - Reference the Authentication section for detailed instructions
  - Link to official PostHog API documentation
- Include specific examples for cloud (app.posthog.com, eu.posthog.com) and self-hosted URLs

* Add PostHog API authentication documentation

- Add Authentication section with step-by-step instructions for obtaining Personal API Key
- Include guidance on determining the appropriate Server URL (US Cloud, EU Cloud, self-hosted)
- Emphasize that POSTHOG_SERVER_URL must include https:// protocol
- Update all secrets descriptions to reference the Authentication section
- Add links to PostHog account settings and official API documentation

Provides users with clear instructions on how to authenticate with PostHog API

* Fix PostHog API page syntax and add to navigation

- Fix TableOfContents array indentation issue that caused acorn parsing error
- Add Authentication section with Personal API Key and Server URL instructions
- Add PostHog API to development section navigation (_meta.tsx)
- Page should now render correctly without 'Could not parse expression' error

* Fix curly apostrophes in PostHog authentication docs

- Replace curly apostrophes (') with straight apostrophes (')
- This was causing 'Could not parse expression with acorn' error
- Fix description to say PostHog API instead of Datadog API

* adding fixed page

---------

Co-authored-by: Francisco Liberal <francisco@arcade.dev>

Author: jottakka

app/en/mcp-servers/development/_meta.tsx

@@ -3,4 +3,5 @@ export default {
   github: "GitHub",
   firecrawl: "Firecrawl",
   "vercel-api": "Vercel API",
+  "posthog-api": "PostHog API",
 };