Move docs into a docs folder

[mansona]

Considering that we're likely going to be writing a lot more documentation over the coming months, I thought it was about time that we move all the random docs hanging around into a nice and neat docs folder 👍

[void-mAlex]

in my own opinion this would make then way less accessible to people, I've gotten to click through other .md docs because I just opened the repo and they were there to click on
there is great value to open find function in the browser and type in addon and find the document related to that from the main page because it's just there, I personally feel there would be more to lose by moving them than there is to gain from organizational purposes
I hear what you're saying about more documentation, but I thought I would offer this other perspective as well

[NullVoxPopuli]

there are also a bunch of places linking to the docs where they are now. -- since we don't have the ability to redirect on GitHub, we'd break all those links.

[ef4]

Discussed this and decided to replace the existing files each with a message "moved to /docs/..." but all future docs can get created in docs.

We also think we should try to maintain an index in the README that points to the various docs to aid discoverability.

[void-mAlex]

also referenced in

embroider/packages/macros/README.md

Line 9 in 3a232cc

The [Embroider package spec](../../SPEC.md) proposes fixing this by making Ember addons much more static. But they will still need the ability to change themselves in certain ways at app compilation time. Hence this package.

[mansona]

fixed 🎉