Help+LibManual: Move all manpage path handling to LibManual

This way, we'll have an easier time changing these paths.

Userland/Libraries/LibManual/Path.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/LexicalPath.h>
+
+namespace Manual {
+
+extern LexicalPath const manual_base_path;
+
+constexpr StringView const top_level_section_prefix = "man"sv;
+
+}