Add read-only and read-write user auth to Nginx recipe

[MmAaXx500]

Proposed changes

It is mentioned at the top of the page of the nginx recipe that there somewhere would be an example about read-only and read-write user authentication. This is missing, so I do like to fill this gap with this.
I have also changed the image used by the password file creation example because registry:2 does not have the htpasswd utility.

Related issues

Related: #11060, distribution/distribution-library-image#106 (and probably fixes these)

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[usha-mandya]

@MmAaXx500 Thanks for the PR. The source files for Registry docs are now in the upstream distribution repository. Could you suggest your changes in https://github.com/distribution/distribution? Thanks. Closing this PR here. Feel free to continue the conversation after I close.

[MmAaXx500]

@usha-mandya Thanks for the info, I'll open my PRs there. If these pages are moved there, I suggest that the "Edit this page" on the right side of the documentation page be changed to point to the new repository.

[usha-mandya]

Thanks @MmAaXx500. We are looking to revamp the Registry docs section to point users to the upstream repo. :)