Add documentation for the LLVM IR dialect

The LLVM IR pass was bootstrapped without user documentation, following LLVM's
language reference and existing conversions between MLIR standard operations
and LLVM IR instructions.  Provide concise documentation of the LLVM IR dialect
operations.  This documentation does not describe the semantics of the
operations, which should match that of LLVM IR, but highlights the structural
differences in operation definitions, in particular using attributes instead of
constant-only values.  It also describes pseudo-operations that exist only to
make the LLVM IR dialect self-contained within MLIR.

While it could have been possible to generate operation description from
TableGen, this opts for a more concise format where groups of related
operations are described together.

PiperOrigin-RevId: 235149136