Add `OP_PAIRCOMMIT` topic page

References:
- [BIP 442](https://github.com/bitcoin/bips/blob/master/bip-0442.md)
- [OP_PAIRCOMMIT as a candidate for addition to LNhance](https://delvingbitcoin.org/t/op-paircommit-as-a-candidate-for-addition-to-lnhance/1216)