Misc fixes

[chrisrlc]

• https://culibrary.atlassian.net/browse/LP-471: Fix heading blocks so that they actually show as h2 elements but can still have additional markdown and formatting applied
• https://culibrary.atlassian.net/browse/LP-484: Display feedback form to logged out users (continues to display for logged in users as well)
• updated joyce theme screenshot now that background color is removed from home page
• https://culibrary.atlassian.net/browse/LP-236: removed jekyll docs and replaced with link to Confluence documentation: https://confluence.cornell.edu/display/exhibits/Curator+Documentation

@mhk33 - Added you as a reviewer to this PR in case you remember whether it was intentional that the feedback form was hidden to logged out users (perhaps for spam reasons?) or if that was a bug.