Change hello world example to be Bitcoin specific

[nud3l]

Is your feature request related to a problem? Please describe.
The current hello world example is taken from the Solidity guides. This works but doesn't show the capabilities of BOB.

Describe the solution you'd like
Change the hello world example to showcase the possibilities of BOB:

• Step 1: Create a smart contract that allows swapping BTC for USDT using the BTC relay
• Step 2: Add account abstraction to the mix so that a user can pay for the EVM-side fees in tBTC/wBTC
• Step 3: Extend the smart contract so that a user can buy an ordinal with USDT.

Additional context
See current example: https://docs.gobob.xyz/docs/build/getting-started/helloworld

[nakul1010]

@nud3l

• In steps 1 & 3 do u want to use btc_marketplace and ord_marketplace contracts under the hood? So the new HelloBitcoin contract will call the above-mentioned contract with the USDT address as a parameter.
• But this doesn't expose the BtcRelay functionality in the HelloBitcoin contract directly.

OR

• A new HelloBitcoin contract can be created which only uses BtcRelay and doesn't have any dependency on the btc_marketplace and ord_marketplace contracts but will be similar to them.

I am also assuming the HelloBitcoin contract to have its repo and not to be in the current repo.

[nud3l]

Create a new HelloBitcoin contract that only has a dependency on the relay (your option 2). The idea is for the code to be as instructive and simple as possible and self-contained.

I think it would be OK to use this repo and put the example in https://github.com/bob-collective/bob/tree/master/src to make sure we keep it up to date with the other code.