Bunch of fixes in introduction

Fix the 'note' admonition graphic

Scaffolding for a proper 'Introduction' chapter

s/(guide|document)/book/ in the introduction

Add footnotes with expanded hyperlink targets in introduction

Remove the commented out legal notice I was playing with

Update ACKs
Add notes to some of the xsl customization parameters
More edits from Henry, wrt: #43

Consolidate some more 'introduction' sections into just paragraphs

Make committing changes to a backing image its own proper section

Add words in the intro about getting help/submitting feedback/units

Add footnote ref in the backing images section

Add footnote about why we use fully qualified paths when using backing images

Fix some wording in discussion about creating basic images