Add linter to check truncate tag in blog posts

tools/checkdocs/checkdocs.go

@@ -68,13 +68,27 @@ func checkFiles(files []string, logf, fatalf func(string, ...any)) {
 			logf("%q: %s", file, err)
 			failed = true
 		}
+
+		// Check for the truncate string
+		if err = verifyTruncateString(b); err != nil {
+			logf("%q: %s", file, err)
+			failed = true
+		}
 	}
 
 	if failed {
 		fatalf("One or more blog posts are not correctly formatted")
 	}
 }
 
+// verifyTruncateString checks that the truncate string is present.
+func verifyTruncateString(b []byte) error {
+    if !bytes.Contains(b, []byte("<!--truncate-->")) {
+        return fmt.Errorf("truncate string not found")
+    }
+    return nil
+}
+
 // extractFrontMatter returns the front matter of a blog post.
 func extractFrontMatter(fm []byte) ([]byte, error) {
 	var in bool

tools/checkdocs/checkdocs_test.go

@@ -62,3 +62,15 @@ func TestVerifyTags(t *testing.T) {
 	err := verifyTags(fm)
 	assert.EqualError(t, err, `tag "documents databases" is not in the allowed list`)
 }
+
+func TestVerifyTruncateString(t *testing.T) {
+	// Test case where truncate string is present
+	b := []byte("This is a blog post. <!--truncate--> The rest of the post...")
+	err := verifyTruncateString(b)
+	assert.NoError(t, err)
+
+	// Test case where truncate string is not present
+	b = []byte("This is a blog post without a truncate string.")
+	err = verifyTruncateString(b)
+	assert.EqualError(t, err, "truncate string not found")
+}