[DOCS] Add documentation for FungibleTokenMetadataViews

[sisyphusSmiling]

Issue To Be Solved

Expand on existing FungibleTokenMetadataViews documentation with more context and simple examples.

Context

Related: onflow/flow-core-contracts#369

[joshuahannan]

Are you thinking that this should be in the repo's README, or in a page on the docs site?

[sisyphusSmiling]

I wrote this thinking about the docs site - the only info I could find in dev portal is found here and it's pretty sparse in comparison to the README. I think it'd be helpful to have more info about the standard in the docs site + worked into existing tutorials.