[Bot] Revive prdoc bot (#5648)

Prdoc bot was deleted in
https://github.com/paritytech/polkadot-sdk/pull/5457 just after being
added in https://github.com/paritytech/polkadot-sdk/pull/5331 without
replacement.
Now bringing it back until it is integrated into the new command
structure.

Formatting is now also fixed, such that the title is always first and
the description renders correctly.

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>