feat(AEP-5): add AEP around designing an API

[toumorokoshi]

To help make the AEPs easier to consume, as well as provide a starting point for those who would like a from-basics starting point on API design, provide a how-to guide.

[rofrankel]

A few additional thoughts:

1. Should there be something about common variants of the standard methods, e.g. whether they're long-running?
2. This is not currently a complete guide. It's definitely a useful description of part of what has to be done, but there are a bunch of other steps too (maybe just protobuf- or OAS-specific stuff). Might be useful to have a section on that, so that someone can easily get from zero to one by following this guide. (I recognize that this goes beyond the scope of API design, but I think it would make it a more useful and complete document for the target audience.)
3. At Roblox, we have an API design doc template. Should we provide something like that here? It's been really useful for getting design discussions to start off in the right "shape".
4. A small but complete example API (e.g. @makahmad's Twitter for kids) would be really nice to have here.

[rofrankel]

A few additional thoughts:

1. Should there be something about common variants of the standard methods, e.g. whether they're long-running?
2. This is not currently a complete guide. It's definitely a useful description of part of what has to be done, but there are a bunch of other steps too (maybe just protobuf- or OAS-specific stuff). Might be useful to have a section on that, so that someone can easily get from zero to one by following this guide. (I recognize that this goes beyond the scope of API design, but I think it would make it a more useful and complete document for the target audience.)
3. At Roblox, we have an API design doc template. Should we provide something like that here? It's been really useful for getting design discussions to start off in the right "shape".
4. A small but complete example API (e.g. @makahmad's Twitter for kids) would be really nice to have here.

Filed #96 to capture this.

[toumorokoshi]

I forced-merge this due to some of the comments being on commits that no longer exist due to force-push.