docs(v2): update installation docs

[iamayushdas]

Motivation

I was upgrading the docusaurus and saw that new release has been done, and i checked the installation is not updated

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!)

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[netlify]

✔️ [V2]

🔨 Explore the source changes: 54b2213

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60da0e09271b6d0008a65b78

😎 Browse the preview: https://deploy-preview-5073--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	73
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5073--docusaurus-2.netlify.app/

[Josh-Cena]

In fact the website is not incorrect:

Please update to the latest Docusaurus 2 version shown at the top of the page, not what is shown below.

The version here is only an example.

[iamayushdas]

In fact the website is not incorrect:

Please update to the latest Docusaurus 2 version shown at the top of the page, not what is shown below.

The version here is only an example.

yes, I triggered a problem while updating from 2.0.0beta.0 to 2.0.0beta.2 , documentations not loading at all

[slorber]

Thanks

Unfortunately we can't keep updating the docs on each release, we need a better solution to solve this (like using MDX)