[clang] [analyzer] Add documentation for std::variant checker (PR #76501)

[via cfe-commits]

DonatNagyE wrote:

Note: the docs build check failed with
> /home/runner/work/llvm-project/llvm-project/clang-build/tools/clang/docs/analyzer/checkers.rst:2108:Definition list ends without a blank line; unexpected unindent.

https://github.com/llvm/llvm-project/pull/76501