fix(elisp/org-html-export.el): make "Try it yourself" button only for Lean src block

Fix #13
fix: remove redundant HTML_HEAD blocks from chapters
feat: add handles for resizng panes
add(css/images): add handle images
fix(js/main_live.js): use relative path for bg images
feat(04_Interacting_with_Lean.org): add discussion of print commands, overloading, and coercions
feat(05_Inductive_Types.org): add chapter on inductive types