Improvements to porting page #299

Based of feedback from the mailing list.

• Include a link for getting development environment setup.
• Include step by step example for how to do the port, maybe taking a recently added port as an example and walking through the diff. For this it would be worth doing an ARM board port since RISC-V is almost the exact same process with fewer changes need (things like timer/serial driver go through SBI so they're not necessary to port on RISC-V platforms as one example).
• There should be instructions on how to build a standalone kernel so users can check that the kernel builds before trying to deal with sel4test.
• Maybe a link to Microkit's porting section in the manual.
• Is it worth mentioning at least that x86 platforms should 'just work' in case someone is wondering about that?

These are things I can think of right now, I will add to this list (or others can) if more comes to mind.