Revisit README.md to make overview sections more relevant

[braddf]

Part of transitioning this repo from original "meta-repo" for the nowcasting project to a more frontend-focused repo.

Detailed Description

At present, the sections of the README relating to the rest of the OCF codebase, e.g. ML, are written in general project terms from the birds-eye perspective of OCF, which potentially reads slightly oddly for what is otherwise purely a repo containing code for apps. Hopefully we can keep the overview, but make more relevant and sensible for any Typescript-types that stumble across this repo and would like to contribute, to give them a good quick-start on how/what goes on at OCF.

[peterdudfield]

It feels like a lot of the readme is still not needed. I know this is copied over from the meta repo