ci: add multi-os test, lint, and wheel build workflows

Also fixes the sdist leaving wheels empty, pins subby to its git
source in wheel metadata, and fixes docstrings that broke mkdocs --strict.

.github/workflows/docs.yml

@@ -8,13 +8,31 @@ on:
       - "overrides/**"
       - "mkdocs.yml"
       - ".github/workflows/docs.yml"
+  pull_request:
+    paths:
+      - "docs/**"
+      - "overrides/**"
+      - "mkdocs.yml"
+      - ".github/workflows/docs.yml"
   workflow_dispatch:
 
 permissions:
   contents: write
 
 jobs:
+  check:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.x"
+      - run: pip install -e . -r docs/requirements-docs.txt
+      - run: mkdocs build --strict
+
   deploy:
+    if: github.event_name != 'pull_request'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4