Make navigation easier

[Tzky]

I think some Issues currently located in /Spongedocs should belong here as they're design-related. This issue is derived from /SpongeDocs (see: SpongePowered/SpongeDocs#115 )

original Post by @Brycey92:
I propose some indicator, on the left-hand menu, to let people know that there are additional pages under a category. A tree could even be implemented so users can manually expand and hide categories within the menu.

[phase]

Allowing users to view categories they don't have open would be nice.

[stephan-gh]

@phase That sounds like something that needs to be suggested/changed in the RTD Sphinx theme.