Improve documentation/example of the BTC Builder APIs

[ShookLyngs]

Currently, we provide several BTC Builder APIs:

• sendBtc: Send BTC to one or more targets with ease
• sendUtxos: Specify inputs and outputs of the transaction as you like
• sendRgbppUtxos: Construct an isomorphic RGBPP transaction from a RGBPP_CKB_VIRTUAL_TX
• sendRbf: Resend a BTC transaction, providing higher fee rate and total fee to replace the original transaction

These APIs are designed to construct BTC transactions for various purposes, and each one is unique. However, the documentation for each API is still missing, and the examples shown in the README are not sufficiently detailed.

To improve this, we can possibly make the following changes:

1. Write .md documentation for each BTC Builder API, describing each property's purpose, default value, and more.
2. Rewrite the examples provided in the btc README to improve the readability of the example code.

[duanyytop]

I am confused that the sendXXX functions give the first impression that they will build BTC transactions and broadcast them, but they only build transactions.

Does anyone else have similar confusion?

[ShookLyngs]

I am confused that the sendXXX functions give the first impression that they will build BTC transactions and broadcast them, but they only build transactions.

Does anyone else have similar confusion?

We have sendX and createSendXBuilder APIs in pairs, so maybe createSendXPsbt is a more appropriate naming pattern.