Add 0.3 Tagged Hashes

[bitschmidty]

• adds 0.3 notebook for tagged hashes
• removes introductory tagged hash content from 2.3
• adds reference to 0.3 in README

Addresses https://github.com/bitcoinops/taproot-workshop/issues/91

[jnewbery]

Thanks for doing this, @bitschmidty. I've made a couple of minor changes. Let me know what you think.

[jnewbery]

I've squashed all commits. Thanks again @bitschmidty !