Add lab instructions

[vityaman]

At least copy-paste that .pdf files here. Maybe rewrite them to markdown.

Needed docs:

• [ ] Installation and first boot guide.
• [ ] Development environment guide.
• [ ] Lab 1: ping-pong instructions.
• [ ] Lab 2: memalloc instructions.
• [ ] Lab 3: cow-fork instructions.

[vityaman]

I have it at my repo https://github.com/vityaman-edu/os-xv6-riscv/tree/dev/doc

[e1turin]

I have some useful comments for every lab in my repo https://github.com/e1turin/itmo-os-xv6-riscv/