docs: renamed Webiny Applications menu to Serverless CMS & added the extend GraphQL API guide to menu

[swapnilmmane]

Thanks: Adrian & Pedro for the inputs.

Short Description

Renamed Webiny Applications menu to Serverless CMS.

Also added the extend GraphQL API guide to the menu again.
I removed it while menu restructuring; I was not sure that this would be relevant after our new scaffold. I discussed this with @doitadrian, and we concluded it is still relevant and gives way to users who manually want to extend GraphQL API with lesser code, thus added it back.

Relevant Links

https://deploy-preview-362--webiny-docs.netlify.app/docs

Checklist

N/A

• I added page metadata (description, keywords)
• I've added "Can I Use This?" section (if needed, e.g. if documenting a new feature)
• I added What You'll Learn at the top of the page and every item in the list starts with a lower case letter
• I used title case for titles and subtitles (in the main menu and in the page content)
• I checked for typos and grammar mistakes
• I added alt / title attributes for inserted images (if any)
• When linking code from GitHub, I did it via a specific tag (and not next / v5 branch)

Screenshots (if relevant):

N/A