docs: Create google analytics doc

[oluwatobiss]

Motivation

This PR creates a Google Analytics doc showing how to add Google's Global Site Tag (gtag.js) to a Nextra website.

Issue

Closes #2128

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Latest commit: 75532d998c8a7fead469c32631d41e6ba5e7dd34

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The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
nextra	❌ Failed (Inspect)			Sep 25, 2023 10:18pm
nextra-v2	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 25, 2023 10:18pm

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[oluwatobiss]

All done.

[oluwatobiss]

Thank you for writing this. Works great.

Here's some unsolicited comments changing everything to typescript since the rest of the project is ts.

Thank you, @basedMcghee, for your helpful suggestions. Having TypeScript examples makes sense. However, we don't need to replace the JavaScript. Instead, we can document the two options so that users who do not wish to use TypeScript can still follow along with the JavaScript examples.

[ziegenbalg]

I've tried these changes and get a syntax error:

 1 │ export const GA_TRACKING_ID = process.env.NEXT_PUBLIC_GA_ID
 2 │ export const pageview = (url: string) => {
   ·                             ─
 3 │   window.gtag('config', GA_TRACKING_ID, {
 4 │     page_path: url
 5 │   })
   ╰────

Caused by:
    Syntax Error

Import trace for requested module:
./lib/gtag.js
   automatically enabled Fast Refresh for 3 custom loaders
 ⨯ ./lib/gtag.js
Error: 

[cenobitedk]

This guide is specifically designed to work with Vercel, but it doesn't work under different conditions. lib/gtag.js is inlined during build-time, not at runtime, so this would only work if the image is build on the server. In many cases this is not true. Image is often build on CI (for multiple environments) where the env var is not available, which it won't be because you don't want to build different images for different environments.