doc: New documentation styling

.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

doc: Removed unused template