doc: lint markdown files

Only way to have a unified markdown syntax is to enforce the wanted
syntax by linting the files. Don't rely on the many markdown syntaxes,
be consistent.

scripts/markdown-lint.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+
+## SPDX-FileCopyrightText: 2024 Benjamin Grande M. S. <ben.grande.b@gmail.com>
+##
+## SPDX-License-Identifier: AGPL-3.0-or-later
+
+# shellcheck disable=SC2086
+set -eu
+
+command -v git >/dev/null ||
+  { printf "Missing program: git\n" >&2; exit 1; }
+cd "$(git rev-parse --show-toplevel)" || exit 1
+./scripts/requires-program.sh mdl
+
+find_tool="find"
+if command -v fd; then
+  find_tool="fd"
+elif command -v fdfind >/dev/null; then
+  find_tool="fdfind"
+fi
+
+if test -n "${1-}"; then
+  files=""
+  for f in "$@"; do
+    test -f "$f" || continue
+    extension="${f##*.}"
+    case "$extension" in
+      md) files="$files $f";;
+      *) continue;;
+    esac
+  done
+  test -n "$files" || exit 0
+  exec mdl ${files}
+fi
+
+case "${find_tool}" in
+  fd|fdfind) files="$(${find_tool} . --extension=md)";;
+  find) files="$(find minion.d/ -type f -name "*.md")";;
+esac
+
+exec mdl ${files}