ci: add link checker, README convention check and PR templat

.github/scripts/check_readme.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""Check README.md resource entries against the conventions in CONTRIBUTING.md."""
+import re
+import sys
+from pathlib import Path
+
+README = Path(__file__).resolve().parents[2] / "README.md"
+
+# Type marker, then optional beginner (👶), then optional invaluable (💎).
+ENTRY = re.compile(r"^- \[(?:📘|🎞️?|📝|🔗|🎧)👶?💎? .+\]\(https?://\S+\)")
+LIST_LINK = re.compile(r"^- \[")
+URL = re.compile(r"\]\((https?://[^)\s]+(?:\([^)\s]*\))?[^)\s]*)\)")
+BAD_CHARS = {"\ufffd": "U+FFFD replacement character", "\u200b": "zero-width space",
+             "\u200c": "zero-width non-joiner"}
+HEADING = re.compile(r"^#{2,6} ")
+
+
+def main() -> int:
+    errors = []
+    section = ""
+    seen = {}
+    for number, line in enumerate(README.read_text(encoding="utf-8").splitlines(), 1):
+        for char, name in BAD_CHARS.items():
+            if char in line:
+                errors.append(f"{number}: {name}")
+        if HEADING.match(line):
+            section = line
+            seen = {}
+            continue
+        if not LIST_LINK.match(line):
+            continue
+        if not ENTRY.match(line):
+            errors.append(f"{number}: entry does not match '- [<type>[👶][💎] Title](url)'")
+            continue
+        match = URL.search(line)
+        if match:
+            url = match.group(1)
+            if url in seen:
+                errors.append(f"{number}: duplicate URL in section, first used on line {seen[url]}")
+            seen[url] = number
+    for error in errors:
+        print(f"README.md:{error}")
+    return 1 if errors else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())