Use bootstrap-based template for API reference

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add theme flag to build_pelican_doc

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Link to NuvolaKit API reference in doc index

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
doc template: more margin for large screen

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>