changed theme to sphinx_rtd_theme

Signed-off-by: David Wosnitza <david@druul.in>
exchanged colors

Signed-off-by: David Wosnitza <david@druul.in>
fixed indentation/linkref errors

Signed-off-by: David Wosnitza <david@druul.in>
reverted changes on source/index.rst

Signed-off-by: David Wosnitza <david@druul.in>
rebuild and optimized toctrees

Signed-off-by: David Wosnitza <david@druul.in>
added original license to theme
increased top and bottom margins for printing

Signed-off-by: David Wosnitza <david@druul.in>
increased bottom margin

Signed-off-by: David Wosnitza <david@druul.in>
[ci skip] Remove an unused lang string

Close #3411
Merge pull request #3410 from druu/feature/userguide_redesign

Feature/userguide redesign