move style guide into repository
Moves the wiki page https://gitlab.mpi-sws.org/iris/iris/-/wikis/style-guide into the repo, so that we can use the usual MR process for style guide changes.
Moves the wiki page https://gitlab.mpi-sws.org/iris/iris/-/wikis/style-guide into the repo, so that we can use the usual MR process for style guide changes.