Change default folder to wiki

[jcbhmr]

⚠️ this is a breaking change.... BUT i think that's ok since we are still at v0 with specific tags i.e. no risk of a v0.2 (which doesn't exist btw) suddenly breaking

this PR would... checkboxes track draft

• change readme option text to match
• change action.yml default value
• change action.yml text
• OPTIONAL: change example to be non-default? like "my-wiki" or something?
• move test folder from .github/wiki to wiki/

discussed in #33

biggest proponent:

a point in favor of wiki vs .github/wiki is its prevalence in existing repos and the convention of where wiki docs are put: 137,000 in /wiki/**.md vs 154 in /.github/wiki/**.md -- pretty stark difference 🤪

[jcbhmr]

@spenserblack ready for review

[spenserblack]

I think perhaps we should see what other users do. Do they often override the default folder to be /wiki/? I have a weirdly strong opinion about .github/wiki, but I'll change my mind if I'm obviously outvoted 🙂

[jcbhmr]

I can tell this is going nowhere with #81 in progress 😅