feat: add demo spec

[wischli]

Description

• Adds demo spec which enables syncing with Demo chain from scratch with latest docker image
• Without this PR, demo can only be synched via centrifugeio/centrifuge-chain:demo-genesis-20220801173354-feffc7c

Generated via

1. Build spec from docker image which is able to create genesis state

docker run --platform=linux/amd64 centrifugeio/centrifuge-chain:parachain-20220801173354-feffc7c build-spec --chain=development --disable-default-bootnode > demo-spec.json 

2. Change hardcoded parachain id

      "parachainInfo": {
-        "parachainId": 10001
+        "parachainId": 2031
      },

3. Copy spec into docker image

# Dockerfile
FROM centrifugeio/centrifuge-chain:parachain-20220801173354-feffc7c
COPY ./demo-spec.json /resources/demo-spec.json

4. Generate raw spec

docker run --platform=linux/amd64 tmp-demo-genesis build-spec --chain=/resources/demo-spec.json --disable-default-bootnode --raw > demo-spec-raw.json 

Checklist:

• I have added Rust doc comments to structs, enums, traits and functions
• I have made corresponding changes to the documentation
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works

[NunoAlexandre]

Thanks for all the context in the description, this is good built-in documentation 👌