docs: 'BskyAgent' is deprecated

[adamclark64]

WHAT: update get-started.mdx docs

WHY:

'BskyAgent' is deprecated.ts(6385)
bsky-agent.d.ts(2, 5): The declaration was marked as deprecated here.

[bamnet]

AtpAgent is planned to be deprecated soon too. The inline docs recommend jumping straight to Agent / CredentialSession like:

const session = new CredentialSession(new URL("https://api.bsky.app"));
  
export const agent = new Agent(session);

(regardless, this feels like one of the more impactful PRs to merge since it's in the getting started flow)

[adamclark64]

AtpAgent is planned to be deprecated soon too. The inline docs recommend jumping straight to Agent / CredentialSession like:

const session = new CredentialSession(new URL("https://api.bsky.app"));
  
export const agent = new Agent(session);

happy to make this change if it makes more sense