📚🔗 Broken links on docs index page

[fwkoch]

The index file in the docs (https://github.com/executablebooks/thebe/blob/main/apps/docs-core/index.md) has a few broken links to using-thebe-* pages. Looks like there was some toc rearrangement to improve the docs flow, commenting out the linked pages: https://github.com/executablebooks/thebe/blob/main/apps/docs-core/_toc.yml#L41-L44

I tried to put together a simple PR, but there wasn't quite a 1:1 drop-in replacement for the missing links.

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