deny missing docs on public items

[sam0x17]

Right now we actually have a lot of // comments that are intended to be /// doc comments. This is rampant throughout our pallets. A really good way to fix this would be to enforce #[deny(missing_docs)] at the crate level for each of our crates and then work through all the compile errors. Prob a big effort but will dramatically improve @rajkaramchedu's effort to improve the subtensor docs, and will require us to document things going forward as we create them.

AC:

• add #[deny(missing_docs)] to all subtensor crates
• fix the resulting compile errors, upgrading any // item-level comments to /// and ensuring they do not introduce any broken doc links
• cargo doc --workspace passing without any broken doc links