Add extended documentation about PersistentVolumes within a PipelineRun

[jlpettersson]

Changes

There are many things to think about when using PersistentVolumes within a PipelineRun.
We have introduced the Affinity Assistant to avoid them. This documentation intend to better
describe those problems and configurations so that an end-user can do better choice,
e.g. when disabling the Affinity Assistant.

This is things that have been discussed in previous PRs, e.g. #2630 (comment) and #2885 (comment)

/kind documentation

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[ghost]

Hey thanks for this, it's much clearer to me now how the volume configuration and affinity assistant tie together. I left a bunch of language style/nit comments but nothing that is blocking.

[vdemeester]

/lgtm

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[bobcatfish]

Thanks for adding this @jlpettersson !!! ❤️