This PR adds description to each of the sections of the Changelog part.
Changes are based on feedback that it wasn't that clear what exactly
`Node Dev`, `Runtime Dev` etc. means. Now, the description for each of
those parts is taken directly from the `prdoc` schema.
Closes https://github.com/paritytech/release-engineering/issues/197

This PR introduces the github flow which will create a release draft
automatically when the rc tag is pushed. The flow contains the following
steps:

- Gets the info about rust version used to build the node
- Builds the runtimes using `srtool`
- Extracts the info about each runtime 
- Aggregates the changelog from the prdocs
- Creates the release draft containing all the info related to the
release (changelog, runtimes, rust versions)
- Attaches the runtimes to the draft
- Posts the message to the RelEng internal channel to inform that the
build is done.

Related to the #3295

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Co-authored-by: Oliver Tale-Yazdi <[email protected]>