Parse table of contents from json

After #253 , enhance Tutorial Template markdown rendering to parse table of contents field.

Originally posted by @kanishka-work in #252 (comment)

Would be nice to improve the bindings for the selected library to return a result instead of throwing an exception when there is a parse failure.

Review md-ast and other standards from unifiedjs and remark, while implementing this. We should use the same general data types. https://github.com/syntax-tree/mdast-util-toc

If we continue to need this issue because we need to transfer generated table of contents from an ocaml process to a rescript process, then revisit #253 and introduce a json decoding library.

If omd generates the table of contents, then this task will be necessary (unless we compile omd with jsoo). If remark generates the table of contents, then the values will remain in memory and there won't be any need to serailize to and deserialize from json.

Most likely, remark will be generating the table of contents, so there will not be a need for deserializing from json. #243 tracks the actual work that remains.