Publish ledger specifications as CIPs and explicitly endorse the process

The ledger is obviously a critical part of the Cardano as it embodies most of the rules governing how various parties interact with each other through the network. While there is a `Ledger` category, it seems the ledger project does not explicitly endorse the CIP process, along the lines of what Plutus project did in [CIP-35](https://github.com/cardano-foundation/CIPs/tree/master/CIP-0035). Following revised CIP-0001, there's now explicitly a section on project's endorsement: https://github.com/cardano-foundation/CIPs/tree/master/CIP-0001#project-enlisting

Also, it seems to me it could be worthwhile to publish all ledger specifications, past, present and future as CIPs, effectively replacing this list of PDFs/CDDLs

by a list of CIPs.