Define a base type for ThemeConfig instead of any

[kiaking]

Right now this break features like next and previous links with locales because the sidebar is supposed to be an array while it's supposed to be an object on locales but next and previous links do not support that format and TS is not picking up any problem.

If sidebar items is supposed to be defined outside of themeConfig, then move it out and expose it as a type. It can also be used inside locales in SiteData type