Document the meaning of writeset and commit evidence

[letmaik]

Those should be documented since they are exposed in receipts.

[achamayou]

• Add Write Set entry to the glossary pointing to https://microsoft.github.io/CCF/main/architecture/ledger.html?highlight=writeset#transaction-format
• Add Commit Evidence to the glossary and include a short summary of its function (details under Commit evidence in receipts #3295)

Also:

• Receipt architecture page, motivating the high-level goals, linking to Audit and Use Apps pages
• Use Apps: cover receipts as service signatures with optional user claims
• Add detail about cert, identity of signer
• Make explicit the requirement to have the service identity
• Add a commented sample use of custom claims in the JS logging app, showing how to expose the log message directly in the receipt
• Audit: cover receipts as audit elements, potentially to be combined with a ledger, move to first position
• Add a commented sample use of a receipt with a ledger or online service to detect and blame a malicious catastrophic recovery (equivalent to the next item)
• Add a commented sample use of a receipt + ledger to check for inclusion

[achamayou]

Started under https://github.com/achamayou/CCF/tree/receipt_doc_changes, will tick off as PR'ed.