Tokens in FRAME Docs (#2802)

Closes https://github.com/paritytech/polkadot-sdk-docs/issues/70

WIP PR for an overview of how to develop tokens in FRAME. 

- [x] Tokens in Substrate Ref Doc
  - High-level overview of the token-related logic in FRAME
- Improve docs with better explanation of how holds, freezes, ed, free
balance, etc, all work
- [x] Update `pallet_balances` docs
  - Clearly mark what is deprecated (currency)
- [x] Write fungible trait docs
- [x] Evaluate and if required update `pallet_assets`, `pallet_uniques`,
`pallet_nfts` docs
- [x] Absorb https://github.com/paritytech/polkadot-sdk/pull/2683/
- [x] Audit individual trait method docs, and improve if possible

Feel free to suggest additional TODOs for this PR in the comments

---------

Co-authored-by: Bill Laboon <laboon@users.noreply.github.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>