rework docs for POSIX shared memory
rework docs for POSIX shared memory

- replace json w/ yaml
- omit superfluous "running"
- spell out "infrastructure"
- add filename title and ‘source’ directive
- update cmd accordingly
- change **** blocks to ---- blocks
- markup /dev/shm
- use "provide" instead of "mount in"
- add intro blurb for example
- add callouts
- add "oc create" intro blurb

xiaoping378/openshift-docs,xiaoping378/openshift-docs,xiaoping378/openshift-docs