docs: Add PostHog plugin

[ivanagas]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.

Motivation

PostHog has a plugin that makes it easy to add analytics, session replays, surveys, and more to Docusaurus. I just updated it and the docs for it.

The Google Analytics plugin is also being deprecated, meaning there will be fewer options for analytics.

Test Plan

Tested plugin locally and it is already in use.

Test links

https://docusaurus.io/docs/api/plugins

Deploy preview: https://deploy-preview-10163--docusaurus-2.netlify.app/docs/api/plugins/

Related issues/PRs

PostHog/posthog.com#8512
PostHog/posthog-docusaurus#15

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 67	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 53	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 70	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 70	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report

[slorber]

Hi @ivanagas 👋 funny to see you there.

Just realized that there's an official posthog plugin that I could use on my own newsletter website instead of a custom posthog-js integration 😅

But apparently this was already added on our community resources pages a while ago (#2957)

The docs page you modify is only for listing official first-party plugins that we build ourselves and maintain as part of this monorepo, so I'm going to close

[ivanagas]

Ahh, missed this. Thanks!