docs: link to migration guide from Blogger to Docusaurus / remove blog.johnnyreilly.com Google Analytics tag from docs

[johnnyreilly]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

I'm really pleased that I migrated from Blogger to Docusaurus. But it wasn't straightforward. I want to make it easier for others to follow the same journey.

Also, it turns out people have been copy / pasting the example GTAG config from the docs which features my Google Analytics tag: https://docusaurus.io/docs/api/plugins/@docusaurus/plugin-google-gtag#ex-config

As a consequence, these websites are sending traffic to my Google Analytics property. My life is now spent sending PRs like this:

• fix: remove blog.johnnyreilly.com Google Analytics tag novuhq/novu#1992
• https://github.com/undistro/undistro.io/pull/80
• fix: stop sending google analytics traffic to blog.johnnyreilly.com dgiot/dgiot_doc#74

I'd like to make it less likely that others make the same mistake.

If you're curious how I realised: https://twitter.com/johnny_reilly/status/1590585546451333120

Test Plan

N/A

Test links

Deploy preview: https://deploy-preview-8313--docusaurus-2.netlify.app/community/resources#articles

Related issues/PRs

#8293

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 58	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 75	🟢 100	🟢 100	🟢 100	🟢 90	Report

[johnnyreilly]

Thanks!

[slorber]

You may want to edit older docs versions too to remove it everywhere 😅