Ref docs for `frame_currency`

[juangirini]

Create the reference docs as part of the Developer Hub -> https://github.com/paritytech/polkadot-sdk/blob/9c2b3eaa8c05f4f25809e87f1890679261cd1e03/developer-hub/src/reference_docs/mod.rs#L70

The focus should be: Learn about the currency-related abstractions provided in FRAME.

Notes:

• History, Currency trait.
• Hold and Freeze with diagram.
• HoldReason and FreezeReason
• This footgun: Ensure correct variant count in Runtime[Hold/Freeze]Reason polkadot-sdk#1900 (comment)

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/decentralized-futures-ecosystem-devrel-team-for-polkadot-by-papermoon/5811/7