[k8s] Clarified the usage of shared memory.

[jsuchome]

Added extended shared memory for default kubernetes manigest. Also, point various cache directories to writable /tmp so that pod does not need to be run as root.

Motivation

Added some more clarification for the usage of shared memory increase. It's necessary for single node installation too, if one wants to use some parallelism techniques. See #4259

Also, k8s pod does not need to run with root security context, but it needs some tinkering.

Modifications

• small update on the parallelism docs page
• extended basic k8s manifest to include increased shared memory volume
  - additionally added env variables to enforce creating caches in writable volume, so that the pod can be run as non-root

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