Theme: switch to agogo

- switch to agogo theme
- move HTML options section
- change logo
- set header color to grey
- deactivate show source and index
- deactivate custom paths to prevent build warnings
- do not show project version
- change HTML title
- add favicon and new logo files

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>