github: generate documentation version matrix dynamically

Updating the release tag in docs.yml is tedious. Generate the action
matrix dynamically.

.github/config.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+# Copyright (c) 2023, Jani Nikula <jani@nikula.org>
+# Licensed under the terms of BSD 2-Clause, see LICENSE for details.
+
+# Output a json matrix for building multiple versions of documentation
+
+# Can't use git describe unless full history has been fetched!
+release=$(git tag --list --sort=version:refname | tail -1)
+
+jq -c . <<EOF
+{
+  "include": [
+    {
+      "ref": "master",
+      "name": "dev"
+    },
+    {
+      "ref": "$release",
+      "name": "stable"
+    }
+  ]
+}
+EOF

.github/workflows/docs.yml

@@ -6,16 +6,25 @@ on:
       - 'master'
 
 jobs:
+  configure:
+    name: Configure Versions
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.version-matrix.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v3
+      - name: Fetch tags
+        run: git fetch --tags origin
+      - name: Set version matrix
+        id: version-matrix
+        run: echo "matrix=$(./.github/config.sh)" | tee $GITHUB_OUTPUT
+
   build-docs:
     name: Build Documentation
     runs-on: ubuntu-latest
+    needs: configure
     strategy:
-      matrix:
-        include:
-          - ref: master
-            name: dev
-          - ref: v0.15.0
-            name: stable
+      matrix: ${{ fromJson(needs.configure.outputs.matrix) }}
     steps:
       - uses: actions/checkout@v3
         with: