Split documentation for Elements/Bitcoin/Unchained

[schoen]

Related to #37.

All documentation needs to be reviewed to ensure it's applicable to each deployment. The biggest difference is in jets (both the jet documentation itself and any code examples that make use of jets, e.g. to check asset types).

We have a built-in feature in Material for MkDocs where we can create tabs like

=== "Elements"

    Documentation for Elements integration goes here.

=== "Bitcoin Signet"

    Documentation for Bitcoin integration goes here.

This works great.

Asking DeepWiki about pages that are Elements-specific also seems to return a thorough answer. It's not too bad so far. Also, git grep jet:: in the repository doesn't show very many code examples that use Elements-specific jets at present.

[schoen]

The only code examples that I've found that appear on the docs site directly and that needs a change are the covenant examples (last-will and third-time) because of the fee payment (jet::output_is_fee()). This is different because fees are always implicit in Bitcoin and explicit in Liquid. The Bitcoin versions can have the same underlying covenant logic, just assert that this output does not exist at all, rather than being a fee output.