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>

prdoc/schema_user.json

@@ -17,6 +17,16 @@
         "type": "string",
         "description": "Title for the PR. This is what will show up in the release notes.\nif needed, you may provide a different title override for each audience in the `doc` property."
       },
+      "author": {
+        "title": "Author handle",
+        "type": "string",
+        "description": "Author handle"
+      },
+      "topic": {
+        "title": "Topic",
+        "type": "string",
+        "description": "Topic"
+      },
 
       "doc": {
         "type": "array",