Add user guide for direct usage with containerit, o2r-meta, and erc-checker

[nuest]

Something along the lines of "this is how you can check before uploading if we can create a runtime manifest (using containerit) and what kind of metadata we will be able to extract (with o2r-meta extract)".

[ghost]

what kind of metadata we will be able to extract (with o2r-meta extract)

Does this fit in the template section (e.g. geo template), where yaml headers are prepared that can be filled out in a workspace's rmd?

[nuest]

Yes, it's related to the templates and the content should be linked.