Support "in-place" lists of contents

[chrisjsewell]

I think the best way forward is to use the simple lists that @AakashGfude has put together and we can open an issue to replace it with a more pure latex implementation using some latex package to generate simple local contents listings. @AakashGfude can you open a PR to add in this support.

this should not be that difficult:

1. The point of this PR, and thus any future work, is that for builder specific extensions you can "gather" information in a transform, but you cannot implement any modifications of the AST until a post-transform.
2. The list addition must be optional, and should be one option of how to generate an in-page toc.

Originally posted by @chrisjsewell in https://github.com/executablebooks/jupyterbook-latex/issues/55#issuecomment-832304094

[mmcky]

@AakashGfude this is now supported right. We can close this?