Generate a subset of the Design Principles doc for web developers

[LeaVerou]

This is a spinoff from the discussion for #177. A lot of our design principles apply to developer code as well, not just spec authors. E.g. things like JS API design and naming are more broadly useful. It may be good to annotate these principles so that in the future we can generate a version of the design principles doc targeting web developers (akin to the subset of the HTML spec aimed at web developers).

[LeaVerou]

Thankfully, Bikeshed can do that really easily: https://tabatkins.github.io/bikeshed/#conditional

[LeaVerou]

@ylafon and I started going through the principles to see what might be relevant to web developers and what is not, we recorded our work so far in this google doc.

It's a shame that a lot of HTML principles that would be useful to web authors designing web components are not there yet.