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

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

This PR backports:
- node version bump
- `spec_vesion` bump
- reordering of the `prdocs` to the appropriate folder
from the `1.9.0` release branch

This fixes an issue introduced in
https://github.com/paritytech/substrate/pull/14101, in which I removed
the `Call` enum's documentation and replaced it with a link to the
`Pallet` struct, but this also removed any docs related to call from the
metadata.

I tried to add a regression test for this, but it seems to me that this
is not possible, given that using `type-info` we only assert in type-ids
for `Call`, `Event` and `Error`. I removed some doc comments from a test
setup in `frame-support-test` to demonstrate the issue there. @jsdw do
you have any comments on this?

I also fixed a small issue in the custom html/css of `polkadot-sdk-doc`
crate, making sure it does not affect the rust-doc page of all other
crates.

- [x] Investigate a regression test
- [x] prdoc