docs: add documentation about workspace retention rules

[VMois]

Tasks:

• add a page that describes how to use retention rules work from a user perspective
• add a page describing how to set up retention rules for administrators.
• update reana.yaml specification page

[VMois]

Found PR about the workspace part from a while ago - #94. I might use it for some resources regarding root_path as currently, we do not have it in our documentation.