Add a CI script to automate "this month in mathlib"

.github/workflows/make-monthly-pr.yml

@@ -0,0 +1,46 @@
+name: "Create 'This month in mathlib' PR"
+# This workflow will overwrite any currently open PR for the previous month. Do not run it once a maintainer has started curating the month!
+
+on:
+  # Allows us to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+jobs:
+  generate:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          path: blog
+      - uses: actions/checkout@v3
+        with:
+          repository: leanprover-community/mathlib
+          path: mathlib
+
+      - name: Generate the blog post
+        id: generate
+        run: |
+          # get the first day of last month. Hack taken from https://www.gnu.org/software/coreutils/manual/html_node/Relative-items-in-date-strings.html
+          date="$(date +%Y-%m-15) -1 month"
+          date="$(date --date "$date" +%Y-%m-01)"
+          echo "::set-output name=iso-date::$(date --date="$date" +"%Y-%m")"
+          slug_date="$(date --date="$date" +"%b-%Y")";
+          slug_date="${slug_date,,}";  # lowercase
+
+          # fetch all the commits from that date onwards
+          ( cd mathlib;
+            git fetch origin master --shallow-since="$date" );
+
+          cd blog
+          ./monthly-blog.sh "$(date --date="$date" +"%Y")" "$(date --date="$date" +"%-m")" ../mathlib > posts/month-in-mathlib-$slug_date.md
+
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@v4
+        with:
+          branch: this-month-in-mathlib/${{ steps.generate.outputs.iso-date }}
+          title: This month in mathlib (${{ steps.generate.outputs.iso-date }})
+          commit-message: Auto-generated PR log for ${{ steps.generate.outputs.iso-date }}
+          body: |
+            This pull request was generated by a manual trigger of the [`make-monthly-pr`](https://github.com/leanprover-community/blog/actions/workflows/make-monthly-pr.yml) action in this repository. 
+          path: blog