Make yang module html documentation more user friendly.

There are several points where the html documentation can be made more useful:

• module: missing description
• module.imports: one per line, add prefixes
• groupings (especially large ones)
• could be collapsable
• should contain links to subgroupings (if reachable locally) or at collapse by default
• augmentations miss name/identifier
• augmentation target should use prefixes instead of full namespaces (full namespace can be provided via tooltip on pefix)
• general coloring - deprecated elements should use gray color or strikeout.