Docs: re-org and improve getting started docs

[yanyongyu]

resolve #285

[galshi]

I'm not sure why you included this, I don't think someone is likely to encounter this.
Also this is a general python error not something specific to githubkit.

[galshi]

Also, not 100% it's relevant but if someone has a use case where he creates a lot of clients (let's say github app with oauth) you would want a way to open an unknown amount of contexts, although it's also useful for people who just need to open a lot of contexts instead of doing with a, b, c, d...:.

I use something like this:

async with contextlib.AsyncExitStack() as stack:
    for client in client_list:
        await stack.enter_async_context(client)

Might be useful for other people too.