docs: add maintainer's handbook (#5396)
docs: add maintainer's handbook (#5396)

This is a list of recipes used by maintainers to maintain
scylla.git.
avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla