[Docs] Rewrite Agent Structure & Customize Agent

[SaadBazaz]

In "Agent Structure": It would be good to have more examples (with images too) how to customize my agent - for adding features in agents.tsx. Is it possible to see the list of features that I can add to customize my agent?

• Add visual steps and copy-pastable codeblocks
• Clearly describe available features (perhaps in separate sections/pages)
• For default-components.tsx etc aslo it would be good to have some example. In this big manual I found only 1 or 2 times the "Upstreet". Mention Upstreet more (and show somewhere that USDK = Upstreet SDK) in this manual or other documents. This will help associate this usdk product and Upstreet. Also, very useful for SEO

[SaadBazaz]

So we will take Subhani's Personal Assistant Agent as an example to teach a developer how to create custom Components.