Restructure Fully-Typed Contracts section into a How-To Guide

[briwylde08]

What problem does your feature solve?

Fully-Typed contracts (https://soroban.stellar.org/docs/soroban-internals/types/fully-typed-contracts) doesn't entirely fit into the Types section and has more guide content.

What would you like to see?

We'd like to tweak it and move it to the How-To Guides section as "Generate Contract Spec"

[janewang]

@chadoh What do you think of the current page?

[chadoh]

Sorry, missed your comment @janewang! I remember writing this page! It was a long time ago, and there's some outdated content. I've updated that content here:

• Update fully-typed-contracts.mdx #823

I haven't yet formed on opinion on where this content should live at this point.