Replace all todos with issues

[rylev]

The guide is filled with lots of todos. We should ensure that these todos are relevant, have issues, and that the todos references there issues directly in the text.

[igaray]

@rustbot claim

[jyn514]

@igaray FYI I think @rylev has already been working on this a fair bit (at least, they've definitely opened a bunch of issues :laughing:)

[igaray]

Roger! I'll hold off for a bit then.

[rylev]

@igaray feel free to take this if you want. I won't stop you! ❤️ Otherwise I can try to get to it some time this week.