Add source code snippet to demo pages

[rsodre]

Each demo page should have the source code available in a tab or something. At least a link to the demo page in the repository, so we can see how that feature works.

[ooochoche]

@rsodre please is this an open issue

[fracek]

Agree with this. We should have a link to GitHub (so that it's always up-to-date) + a description of what the demo is showing.

[ooochoche]

@fracek can I work on this?

[fracek]

Hey @ooochoche, yes you can work on it.

Follow the following format please:

<Brief description of the demo>

[Link to GitHub](https://github.com/apibara/starknet-react/blob/main/docs/components/demo/<demo-name>.tsx)

For example:

This demo shows how to access the currently connected account and its address.

[Link to GitHub](https://github.com/apibara/starknet-react/blob/main/docs/components/demo/account.tsx)

[ooochoche]

@fracek Thank you. I will get on it.

[ooochoche]

@fracek I want to submit a PR now. Do I need to be assigned first?

[fracek]

You can open a PR and I will review it. No need to have the issue assigned.