Docs restructuring, part 1 (reland)

[theabhirath]

Rebasing #201 was a bit of pain, so I just decided to make a new PR instead. Closes #201 (supersedes).

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This is the PR for landing the first half of the docs restructuring, which will include, amongst other things

1. more docstrings
2. a proper export of the Layers API from Metalhead (or at least the layers that are stable), with documentation added for them
3. Some tutorials to start working with Metalhead (including pre-trained models and using them as feature extractors, as well as a user's guide to working with the mid-level ResNet API).

This is not complete, and needs more work, but I think that can be done in a follow up PR. Some of the work is also dependent on the weights being ported successfully.

[github-actions]

Once the documentation build has completed, you can preview any updated documentation at this URL: https://fluxml.ai/Metalhead.jl/previews/PR221/