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/src/reference_docs/mod.rs

@@ -65,9 +65,6 @@ pub mod metadata;
 /// Learn about how frame-system handles `account-ids`, nonces, consumers and providers.
 pub mod frame_system_accounts;
 
-/// Learn about the currency-related abstractions provided in FRAME.
-pub mod frame_currency;
-
 /// Advice for configuring your development environment for Substrate development.
 pub mod development_environment_advice;
 
@@ -75,6 +72,9 @@ pub mod development_environment_advice;
 // TODO: @shawntabrizi @ggwpez https://github.com/paritytech/polkadot-sdk-docs/issues/50
 pub mod frame_benchmarking_weight;
 
+/// Learn about the token-related logic in FRAME and how to apply it to your use case.
+pub mod frame_tokens;
+
 /// Learn about chain specification file and the genesis state of the blockchain.
 // TODO: @michalkucharczyk https://github.com/paritytech/polkadot-sdk-docs/issues/51
 pub mod chain_spec_genesis;