.gitignore: Removed *.html rule

The Sphinx templates are HTML files and should be included.
doc: Imported Flask Sphinx Styles

from https://github.com/mitsuhiko/flask-sphinx-themes
doc: Activate "Flask" theme
doc: Added proper sidebar to index page
doc: Removed unused themes
doc: Moved links section to its own file
doc: Moved searchbox into searchbox.html file
doc: Moved PDF documentation link into links.html
doc: Removed overwritten searchbox template

The original one works quite well...
doc: Added "Useful Links" section to all sidebars
doc: Adjusted sidebar link
CHANGELOG: Removed trailing whitespace
CHANGELOG: Simplified section headings
doc: Adjusted TOC sidebar section
doc: Moved font family attributes into variables
doc: Use different font combination
doc: Don't show not existing logo
doc: Moved link colors into variables
doc: Use green color for links
doc: Don't use italic font for internal references