fix: Update image paths in design documentation (#1878)

# Goal
The goal of this PR is move Design Docs file from
[LibertyDSNP/DesignDocs](https://github.com/LibertyDSNP/DesignDocs/) to
the `./docs/images` folder and update the references in the
`./designdocs` and `./README.md`

# Checklist
- [ ] Chain spec updated
- [ ] Custom RPC OR Runtime API added/changed? Updated js/api-augment.
- [x] Design doc(s) updated
- [ ] Tests added
- [ ] Benchmarks added
- [ ] Weights updated

designdocs/README.md

@@ -36,7 +36,7 @@ There are three core data models in Frequency and each corresponds to a pallet.
   - The metadata and payload or payload reference that a user sends that matches a particular schema.
   - What and when a message is sent
 
-![Basic Data Model drawio](https://github.com/LibertyDSNP/DesignDocs/blob/main/img/BasicDataModel.drawio.png?raw=true)
+![Basic Data Model drawio](../docs/images/BasicDataModel.drawio.png?raw=true)
 
 ## Frequency Glossary
 

designdocs/message_storage.md

@@ -20,7 +20,7 @@ that will allow us to store these messages on chain.
 Storing messages on chain using **BlockNumber** and **SchemaId** as main and secondary keys
 using [StorageDoubleMap](https://paritytech.github.io/substrate/master/frame_support/storage/trait.StorageDoubleMap.html) data structure provided in Substrate.
 
-![Data-Page-3 drawio](https://github.com/LibertyDSNP/DesignDocs/blob/main/img/main_storage_type.png?raw=true)
+![Data-Page-3 drawio](../docs/images/main_storage_type.png?raw=true)
 
 
 ### Main Storage types
@@ -125,12 +125,12 @@ any messages, to eliminate unnecessary DB reads. We can use a BitArray per Schem
 we would need to store this indexing data off-chain, and we can create jobs to create or update
 it periodically.
 
-![Data-OnChainAnnouncements drawio](https://github.com/LibertyDSNP/DesignDocs/blob/main/img/message_storage_bitvector.png?raw=true)
+![Data-OnChainAnnouncements drawio](../docs/images/message_storage_bitvector.png?raw=true)
 ## Alternatives and Rationale
 Storing messages on chain using a map of `schemaId` and `staring` index to a sequential fixed sized
 bucket.
 
-![Data-Page-2 drawio](https://github.com/LibertyDSNP/DesignDocs/blob/main/img/message_storage_alternative.png?raw=true)
+![Data-Page-2 drawio](../docs/images/message_storage_alternative.png?raw=true)
 
 ### Main Storage types
 - **Messages**