Overhaul content, starting with headings
Add lab section placeholders
Aggregation of similar topics, headings
Overhaul aggregation, content selection of Advanced
Remove intro from Advanced opener
Correct `reset` dup typo
Add Atom Merge Conflict plugin resource
Reorder "Ignore, Cleanup", "GUIs"

* Move “Ignoring” and “Cleanup” sections immediately after
“Collaboration”.
* Relocate “GUIs” section after “Shortcuts”
Add draft content for all lab sections
Merge pull request #221 from github/advanced-content-tuning

Advanced content tuning