Documentation: Placeholders & Todos

[colinoflynn]

Tracking item from #91

fpga-update.md:

• [ ] Do we want to recommend a "user slot"? I specified 'slot 2'. If that is OK remove this todo.
• [ ] Link to bitstream - remove this todo and update URL. Also update wget example if you want to support that.

toolchain-setup.md:

• [ ] Add link to WSL2 PR, tell users to check if that was committed before blindly applying the patch.

building-examples.md:

• [ ] These instructions were written inbetween the build system change - are the environmental variables needed? Update my TODO in addition to fixing docs.
• [ ] Update link to SW Download - maybe tell people to checkout specific tag matching the release is easier? Or do they just download SW code release?

[marnovandermaas]

Most of this is fixed in my PR: https://github.com/lowRISC/sonata-system/pull/94

[colinoflynn]

I was way too slow in the GIT race!