Update analytics doc to use official Gatsby gtag plugin

[meganesu]

Summary

The existing doc about adding Google analytics recommends using the gatsby-plugin-gtag plugin. As was pointed out after #27081 was merged, Gatsby actually maintains an official plugin with the same functionality, gatsby-plugin-google-gtag.

The Adding Analytics doc should be updated to use this official plugin instead.

Steps to resolve this issue

• Update the Google analytics docs to use gatsby-plugin-google-gtag instead of gatsby-plugin-gtag.

Open a pull request

• Open a pull request with your work including the words "closes Update analytics doc to use official Gatsby gtag plugin #27623" in the pull request description

[im-adithya]

I am interested @meganesu. Can you assign this to me? 😄

[uniyalabhishek]

@meganesu Updated the analytics doc to use official Gatsby gtag plugin. Please check PR #27747