Update a few topics for XD

[StefanIvanov]

• Update getting-started
• Most likely remove installing the extension
• Update or delete code extension topic
• Update or delete after code gen

[StefanIvanov]

@DaniMarinov I am tagging you as I feel that in the getting-started we need to add something about app builder

[DaniMarinov]

Yes, I do agree. I will add in the next several days.

[StefanIvanov]

@DaniMarinov if you are done with your updates on the first bullet, you can mark it as complete

[DaniMarinov]

@StefanIvanov the infographic image we've discussed is not visible on staging here.