Wrap Git datastructure diagrams as slides
Tune "Understanding Git" content

Fix typos, table-ize content, revise description sentences.
Update Navigating History descriptions, bash examples
Remove/deprecate Built-In GUIs section

Does not fit within the context of Advanced or provide as much use as
referring to GitHub for Desktop clients.
Rewrite incomplete cherry-pick bullets as paragraph
Tidy cherry-pick command examples
Tidy `checkout` with path description
Rewrite `rebase` descriptions
Correct typos, adjust grammar, remove extraneous

@randomecho provided some great
[insights](https://github.com/github/training-kit/pull/233#discussion-di
ff-17791630) on tuning up the content, all of which have been taken
into account here.
Merge remote-tracking branch 'origin/master' into advanced-format-content-tune-up

Bring latest styling updates and tuning with topic branch
Tidy GPG signing examples
Simplify filter-branch shell examples
Typos, grammer, description tune-ups

Per suggestions:
* https://github.com/github/training-kit/pull/233#discussion_r17945394
* https://github.com/github/training-kit/pull/233#discussion_r17945422
*
https://github.com/github/training-kit/pull/233#discussion-diff-17980309
Merge pull request #233 from github/advanced-format-content-tune-up

Advanced format content tune up