Changelogs local generation (#1411)

This PR introduces a script and some templates to use the prdoc involved
in a release and build:
- the changelog
- a simple draft of audience documentation

Since the prdoc presence was enforced in the middle of the version
1.5.0, not all PRs did come with a `prdoc` file.
This PR creates all the missing `prdoc` files with some minimum content
allowing to properly generate the changelog.
The generated content is **not** suitable for the audience
documentation.

The audience documentation will be possible with the next version, when
all PR come with a proper `prdoc`.

## Assumptions

- the prdoc files for release `vX.Y.Z` have been moved under
`prdoc/X.Y.Z`
- the changelog requires for now for the prdoc files to contain author +
topic. Thos fields are optional.

The build script can  be called as:
```
VERSION=X.Y.Z ./scripts/release/build-changelogs.sh
```

Related:
-  #1408

---------

Co-authored-by: EgorPopelyaev <egor@parity.io>

cumulus/parachains/runtimes/contracts/contracts-rococo/src/lib.rs

@@ -133,7 +133,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("contracts-rococo"),
 	impl_name: create_runtime_str!("contracts-rococo"),
 	authoring_version: 1,
-	spec_version: 1_004_000,
+	spec_version: 1_005_000,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 	transaction_version: 6,