Rewrite README.md

[bluewww]

We should make it more obvious how to quickly run a hello world in RTL sim and on FPGA. Currently, you have to get through a wall of text that talks about a lot of not directly related stuff.

[RenatoBelmonte]

Meanwhile, can you explain which tool to use to compile an application working with the pulp-riscv-gnu-toolchain? The README states (https://github.com/pulp-platform/pulpissimo?tab=readme-ov-file#compiling-applications-for-the-fpga-target) to use PULP-SDK, but it also says that it is not supported (https://github.com/pulp-platform/pulpissimo?tab=readme-ov-file#software-development-kit-unsupported-with-current-release). Maybe I can use FreeRTOS?