Centralize the index and add local navigation functionality

[EssamWisam]

In a previous PR I mentioned that it's not elegant that one has to add a new tutorial to two navigation bars (and modify code instead of data in general). That along with the fact that centralizing the index as a JavaScript object would allow easy implementation of local navigation (which I felt the need for because reaching out for the navigation bar to get the next tutorial takes a little effort) were reasons that made me decide to:

• Centralize the index. It's now a JS object in libs\nav\head.js and any changes to it should meaningfully reflect to both navigation bars. For some reason, it wasn't easy to make the object live in its own file but we can look into this in a later PR where we also consider other parts of file organization in the repo.

• Use the centralized JS index to support for local navigation

local.webm

Don't worry about the undef variables shown in the video. I didn't do an appropriate serve and it surely works as confirmed on fix-minor-plots-validation.