Update/tune Advanced deck section, content, headings

Reorganizes a few topics and groups them into more unified headings. Removes topics yet to be explored/detailed in a manner applicable for Kit.
The heading was removed as the content was relocated to [the beginning](https://github.com/github/training-kit/pull/188/files#diff-bce5e08f883d72fd6f1ce5e6cc296020R31) of topics under [*Understanding Git*](https://github.com/github/training-kit/pull/188/files#diff-bce5e08f883d72fd6f1ce5e6cc296020R9) to unify the topics of data structures, commit fields, then easy was of traversing history.

Seem reasonable?