ignore .clangd

[yangrq1018]

I have a .clangd file like this for my neovim LSP

# patch your C++ standard headers here to make clang happy
CompileFlags:
  CompilationDatabase: .vscode
  Add: [
    -isystem,
    /usr/include/c++/13.2.1,
    -isystem,
    /usr/include/c++/13.2.1/backward,
    -isystem,
    /usr/local/include,
    -isystem,
    /usr/include,
    -isystem,
    /usr/include/x86_64-linux-gnu/c++/13.2.1,
  ]

This file could have some user-specific settings to guide clangd where the standard C++ headers are, it may not be a good idea to track it.