Allow to update `doc/internal/opcode.md` file mechanically from `include/mruby/ops.h` file.
At the same time, the `doc/internal/opcode.md` file has been updated.
Consecutive hyphens at the end of the table were removed as they would have created noise as markdown flavours.
The `xdg-open` command is not available on macOS.
macOS uses the `open` command.
Add info to install doxygen on macOS with Homebrew.
https://formulae.brew.sh/formula/doxygen