Port various Foundation notes as rough draft additions
Add and adjust Git, GitHub slides and supplemental lists
Switch all H3 primary sections to H2

Page title should qualify as semantic H1, logically leading to choice
of all main section headings being H2.
Parse TOC based on heading change

Follows heading format decision in 3633c5c.
Ensure every H2 section opens with slide
Reorg content order, section aggregation
Revert "Parse TOC based on heading change"

This reverts commit 08349f61fd4588ed4cc21eaf279bda3d03278f9d.
Tune style/voice of "chapter" sections
Overhaul entire Foundations

Revised slides, dedicated lab sections, consistent headings
Add "lab" include for special document formatting
Flip header levels back to original (for now)
Add microscope Octicon to lab section headers
Hide "pull" label due to overlap, no build step
Merge pull request #218 from github/class-order-and-content-tuning

Foundations content order and supplemental resource tuning