Retreive Pallet API documentation from Frequency

[wilwade]

Goal: Nice docs that are focused on developers using the chain, not docs for Rust developers

How: Custom Readmes per Frequency Pallet that are structured to also be pulled into these docs

• Templates & Examples
• Capacity Capacity Pallet Documentation Improvements frequency#1997
• Frequency TX Payment Frequency Transaction Pallet Documentation Improvement  frequency#1990
• Handles Handles Pallet Documentation Improvement frequency#1984
• Messages Messages Pallet Documentation Improvement frequency#1981
• MSA MSA Pallet Docs frequency#1962
• Schemas Schemas Pallet Documentation frequency#1968
• Stateful Storage Stateful Storage Pallet Documentation Improvement frequency#1989
• Time Release https://github.com/frequency-chain/frequency/pull/1998
• Add remaining cross links

Generalized Pieces

(Check off once added to ^ not when complete)

• Pallet Errors
  • Frequency
  • Substrate
• Pallet Extrinsics
  • Frequency
  • Substrate
• Pallet State Queries
  • Frequency
  • Substrate
• Runtime RPCs
  • Frequency
  • Substrate
• Node RPCs
  • Frequency
  • Substrate

Old Ideas

• Using the new cargo rustdoc nightly unstable option of --format-output json + something like https://github.com/IonicaBizau/json2md