CppEditor: Tell users about clangd configuration files

It's not feasible to map every possible setting into our UI, so instead
teach people how to use the clangd configuration mechanism.

Change-Id: Id11e81e25b687a4f49af4e090203faca3a75722d
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppcodemodelsettings.cpp

@@ -42,6 +42,7 @@
 #include <QHash>
 #include <QPair>
 #include <QSettings>
+#include <QStandardPaths>
 
 using namespace Utils;
 
@@ -418,6 +419,13 @@ FilePath ClangdSettings::clangdIncludePath() const
     return includePath;
 }
 
+FilePath ClangdSettings::clangdUserConfigFilePath()
+{
+    return FilePath::fromString(
+                QStandardPaths::writableLocation(QStandardPaths::GenericConfigLocation))
+            / "clangd/config.yaml";
+}
+
 void ClangdSettings::loadSettings()
 {
     Utils::fromSettings(clangdSettingsKey(), {}, Core::ICore::settings(), &m_data);