Start consolidating project-wide documentation in the docs/ folder.
While at it, change the name `js-coding-style`, which is both less
ominous and allows for an accompanying `c-coding-style`.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3122>
To avoid having the coding style get out of sync with the actual
coding style in the future, run eslint on the individual snippets
when either the guide or the linter config change.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3146>