Add skills.md to freighter docs

[piyalbasu]

Currently in Freighter's docs, we have some written docs on how to integrate with freighter-api. We often see devs get small things wrong as they're reading quickly or as they're copying/pasting.

We should include a skills.md file that devs can download to take some the guesswork out of how to integrate with either freighter-api or freighter-api via stellar-wallets-kit

As we accumulate these types of skills, it'd be great to centralize these with other skills from the org