Add missing glossary to ref docs (#2572)

This PR is the same as
https://github.com/paritytech/polkadot-sdk/pull/2273, but that one had
too many conflicts after a base change. As it is a just one file PR I
decided to start over with a clean slate.

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This PR adds some missing glossary definitions to the Developer Hub ref
docs

FIxes https://github.com/paritytech/polkadot-sdk-docs/issues/40