Adaptations for current Paclet directory structure; build script and instructions

[felixkasza]

BUILD.md has reasonable build instructions. successinfo.png is an image of a Success[] object, used in BUILD.md. build-jsoupLink.nb builds the paclet. PacletInfo.wl has been adapted to the expectations of MMA 12.3+. The assets/ subdirectory and its contents have moved into Kernel/.

Fixes #4. Fixes #5. Fixes #7.