Underline links on non 'screen' devices

See https://developer.mozilla.org/en-US/docs/Web/CSS/@media#Media_types
for more info
Refactor the menu

- add a button to toggle it
- float menu items for consistency and ease of development (no more need
for specifying how far from the right each icon will be)

Also:
- separate each page's header from the start of the body (that contains
a different menu for different pages)
Add section header/footer comments
Add ascii download

Also: choose different download symbols
Clarify comments
Bugfix code blocks start with an extra space

Paste SRC blocks into the start (not one space to the right) of the
temporary buffer used to highlight them (with pygments).