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>

scripts/release/build-changelogs.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+
+export PRODUCT=polkadot
+export VERSION=${VERSION:-1.5.0}
+
+PROJECT_ROOT=`git rev-parse --show-toplevel`
+echo $PROJECT_ROOT
+
+TMP=$(mktemp -d)
+TEMPLATE_AUDIENCE="${PROJECT_ROOT}/scripts/release/templates/audience.md.tera"
+TEMPLATE_CHANGELOG="${PROJECT_ROOT}/scripts/release/templates/changelog.md.tera"
+
+DATA_JSON="${TMP}/data.json"
+CONTEXT_JSON="${TMP}/context.json"
+echo -e "TEMPLATE_AUDIENCE: \t$TEMPLATE_AUDIENCE"
+echo -e "DATA_JSON: \t\t$DATA_JSON"
+echo -e "CONTEXT_JSON: \t\t$CONTEXT_JSON"
+
+# Create output folder
+OUTPUT="${TMP}/changelogs/$PRODUCT/$VERSION"
+echo -e "OUTPUT: \t\t$OUTPUT"
+mkdir -p $OUTPUT
+
+prdoc load -d "$PROJECT_ROOT/prdoc/$VERSION" --json > $DATA_JSON
+# ls -al $DATA_JSON
+
+cat $DATA_JSON | jq ' { "prdoc" : .}' > $CONTEXT_JSON
+# ls -al $CONTEXT_JSON
+
+# Fetch the list of valid audiences
+SCHEMA_URL=https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+SCHEMA=$(curl -s $SCHEMA_URL | sed 's|^//.*||')
+AUDIENCE_ARRAY=$(echo -E $SCHEMA | jq -r '."$defs".audience.oneOf[] | .const')
+
+readarray -t audiences < <(echo "$AUDIENCE_ARRAY")
+declare -p audiences
+
+
+# Generate a changelog
+echo "Generating changelog..."
+tera -t "${TEMPLATE_CHANGELOG}" --env --env-key env "${CONTEXT_JSON}" > "$OUTPUT/changelog.md"
+echo "Changelog ready in $OUTPUT/changelog.md"
+
+# Generate a release notes doc per audience
+for audience in "${audiences[@]}"; do
+    audience_id="$(tr [A-Z] [a-z] <<< "$audience")"
+    audience_id="$(tr ' ' '_' <<< "$audience_id")"
+    echo "Processing audience: $audience ($audience_id)"
+    export TARGET_AUDIENCE=$audience
+    tera -t "${TEMPLATE_AUDIENCE}" --env --env-key env "${CONTEXT_JSON}" > "$OUTPUT/relnote_${audience_id}.md"
+done
+
+# Show the files
+tree -s -h -c $OUTPUT/

scripts/release/templates/audience.md.tera

@@ -0,0 +1,13 @@
+## Release {{ env.PRODUCT }} {{ env.VERSION }}
+
+Changelog for `{{ env.TARGET_AUDIENCE }}`.
+
+{% for file in prdoc -%}
+#### PR #{{file.doc_filename.number}}: {{ file.content.title }}
+{% for doc_item in file.content.doc %}
+{%- if doc_item.audience == env.TARGET_AUDIENCE %}
+{{ doc_item.description }}
+{% endif -%}
+
+{%- endfor %}
+{%- endfor %}

scripts/release/templates/changelog.md.tera

@@ -0,0 +1,7 @@
+## Changelog for `{{ env.PRODUCT | capitalize }} v{{ env.VERSION }}`
+
+{% for file in prdoc | sort(attribute="doc_filename.number") -%}
+{%- set author= file.content.author | default(value="n/a") -%}
+{%- set topic= file.content.topic | default(value="n/a") -%}
+- #{{file.doc_filename.number}}: {{ file.content.title }} (@{{ author }}) [{{ topic | capitalize }}]
+{% endfor -%}