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>

docs/sdk/Cargo.toml

@@ -66,6 +66,7 @@ pallet-aura = { path = "../../substrate/frame/aura", default-features = false }
 pallet-timestamp = { path = "../../substrate/frame/timestamp" }
 pallet-balances = { path = "../../substrate/frame/balances" }
 pallet-assets = { path = "../../substrate/frame/assets" }
+pallet-preimage = { path = "../../substrate/frame/preimage" }
 pallet-transaction-payment = { path = "../../substrate/frame/transaction-payment" }
 pallet-utility = { path = "../../substrate/frame/utility" }
 pallet-multisig = { path = "../../substrate/frame/multisig" }
@@ -73,6 +74,8 @@ pallet-proxy = { path = "../../substrate/frame/proxy" }
 pallet-authorship = { path = "../../substrate/frame/authorship" }
 pallet-collective = { path = "../../substrate/frame/collective" }
 pallet-democracy = { path = "../../substrate/frame/democracy" }
+pallet-uniques = { path = "../../substrate/frame/uniques" }
+pallet-nfts = { path = "../../substrate/frame/nfts" }
 pallet-scheduler = { path = "../../substrate/frame/scheduler" }
 
 # Primitives