encodings.py: use <th nowrap="nowrap"> for first column

Switch from <td> to <th> to mark this as a heading column; add
nowrap="nowrap" attribute to prevent column from wrapping
encodings.py: oops, errors in HTML header

Sloppy copy/paste, didn't have <head> tags at all (!)
encodings.py: refactor <th> nowrap as CSS

Use a simple CSS style sheet instead of repeated nowrap="nowrap" attributes
encodings.py: add more CSS formatting for tables

Use top-left alignment to make it look less like somebody from
Microsoft's Excel team is trying to make you seasick
encodings.py: Move <hr> down

Somehow it didn't look right.
encodings.py: Put linkified U+F00F instead of repr

Link to fileformat.info for more information about each character and
format it like a proper Unicode U+nnnn character reference in the HTML
rendering
encodings.html: Refresh from latest source