Create a separate doc page on how to build a custom set of images

[mathbunnyru]

Describe your changes

Issue ticket if applicable

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes

[mathbunnyru]

@minrk @consideRatio please, take a look here.

I think it makes it easier to figure out how to build a custom set of images.

[mathbunnyru]

This is how it looks like: https://jupyter-docker-stacks--2144.org.readthedocs.build/en/2144/using/custom-images.html

Note: I'm not a native speaker, so, please, feel free to correct grammar and so on.

[mathbunnyru]

Thanks for such a fast review @manics!

Please, share your opinion, what you think about this PR in general

[mathbunnyru]

Workflow failure is fine - only link check is failing, and, unfortunately, anaconda.org time outs sometimes.

[manics]

Looks good to me!

[minrk]

Nice!