Expand documentation on pinning sessions image

[lorenzo-cavazzi]

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Currently, the few bits of information available for the pinned image used by sessions are available on these pages:

• https://renku.readthedocs.io/en/latest/user/interactive_customizing.html#renku-project-configurations
• https://renku.readthedocs.io/en/latest/user/templates.html?highlight=.dockerignore#renku

The first one should be expanded with a dedicated section and a few extra notes. Remember to update the UI links when that is done.