Update Javascript documentation

[jhofman]

@giorgi-ghviniashvili will update documentation on the javascript side and generate rendered markdown versions. (everything gets written in javascript files, following a few conventions.)

let's also add pointers to everything in https://github.com/microsoft/datamations/tree/main/inst/htmlwidgets/js and add a one-liner pointer in inst/ so people can find this lovely documentation in the future :)

@chisingh, once that's done, can you take a look to review?

[giorgi-ghviniashvili]

Send PR #153 with the new docs. Did not get how to add the link in inst/, let's do it together over the call.

[jhofman]

the markdown documentation looks great, let's also add a high-level summary similar to what sharla did for R here that explains main structure of how to use the app and what goes on at a high-level behind the scenes.

@chisingh, can you take a look once that's done to see how it reads to you and how much you can follow?

[giorgi-ghviniashvili]

@jhofman added the docs here

[jhofman]

this looks good overall, @giorgi-ghviniashvili will add a few notes about tweaks to vega-lite format for datamations

@chisingh notes that at some point we could have testing that tests backend-to-vega spec conversion through javascript. the rest would be more difficult to test because the final output is visual and would likely change over time