chore: remove sip folder and update GitHub URL references

- Remove the sip/ folder from the stacks-core repository as the official SIP documentation is now maintained at https://github.com/stacksgov/sips
- Update GitHub URL reference from blockstack to stacksgov in schema definitions to point to the new location

Closes #5950

Author: orsissimo

docs/rpc/entities/contracts/read-only-function-args.schema.json

@@ -1,7 +1,7 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "title": "ReadOnlyFunctionArgs",
-  "description": "Describes representation of a Type-0 Stacks 2.0 transaction. https://github.com/blockstack/stacks-blockchain/blob/master/sip/sip-005-blocks-and-transactions.md#type-0-transferring-an-asset",
+  "description": "Describes representation of a Type-0 Stacks 2.0 transaction. https://github.com/stacksgov/sips/blob/main/sips/sip-005/sip-005-blocks-and-transactions.md#type-0-transferring-an-asset",
   "type": "object",
   "required": ["sender", "arguments"],
   "properties": {