Add people polkadot genesis chainspec (#5124)

Published as part of the fellowship
[v1.2.6](https://github.com/polkadot-fellows/runtimes/releases/tag/v1.2.6)
release and originally intentionally left out of the repo as the
hardcoded system chains will soon be removed from the
`polkadot-parachain`.

After a conversation in
https://github.com/paritytech/polkadot-sdk/issues/5112 it was pointed
out by @josepot that there should be a single authoritative source for
these chainspecs. Since this is already the place for these it will
serve until something more fitting can be worked out.