docs: Create a markdown-it plugin

docs/gfm-alerts-to-admonitions/tests/test_plugin.py

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+import pytest
+from gfm_alerts_to_admonitions import gfm_alert_to_admonition
+from markdown_it import MarkdownIt
+
+
+@pytest.mark.parametrize("kind", ["tip", "note", "important", "warning", "caution"])
+def test_parse(data_regression, kind):
+    input = f"> [!{kind.upper()}]\n> This is an *alert*"
+    md = MarkdownIt("commonmark").use(gfm_alert_to_admonition)
+    tokens = md.parse(input)
+    data_regression.check([t.as_dict() for t in tokens])
+
+
+def test_case():
+    def make_input(kind):
+        return f"> [!{kind}]\n> tip"
+
+    md = MarkdownIt("commonmark").use(gfm_alert_to_admonition)
+
+    reference = [t.as_dict() for t in md.parse(make_input("TIP"))]
+    lower = [t.as_dict() for t in md.parse(make_input("tip"))]
+    mixed = [t.as_dict() for t in md.parse(make_input("tIp"))]
+
+    assert lower == reference
+    assert mixed == reference