improvement - http reference introduction - add a BskyAgent basic example

[boly38]

(following #62)

starter templates clients explain well how to use BskyAgent agent for basis.
but (for a NooB (like me)) when we arrives on AT Protocol XRPC API (http reference introduction) : it is not easy to understand that this BskyAgent embed natively the api.

I would like to suggest adding this information plus one simple example right here