doc: add link to API documentation on pub.dev

[ptisserand]

Links to API documentation available on pub.dev are missing.

Here are the links to API documentation:

• starknet: https://pub.dev/documentation/starknet/latest/
• starknet_provider: https://pub.dev/documentation/starknet_provider/latest/
• secure_store: https://pub.dev/documentation/secure_store/latest/
• wallet_kit: https://pub.dev/documentation/wallet_kit/latest/
• avnu_provider: https://pub.dev/documentation/avnu_provider/latest/

Link to API should be add in corresponding package documentation page.

[Cedarich]

May I take care of this?

[brainox]

Can I tackle this one?

[mimisavage]

May I take this issue on?

[ptisserand]

May I take care of this?

Sure

[ptisserand]

Link to starknet_provider pub.dev documentation added with #464