Update main docs for downloading booty when ready.

[gerardwebb]

https://github.com/amplify-edge/main/blob/master/doc/user-walkthrough.md#download

https://github.com/amplify-edge/main/blob/master/doc/developer-walkthrough.md

@alexadhy
Just a note to remember to tell a user and dev to use booty, once things are driven by booty and settle down.
We could add extra info about how to do it for Org-y.

Main is the best place still to add all this, rather than booty i think.

[alexadhy]

https://github.com/amplify-edge/sys-share/blob/21c9d79819467c9439e346889d832d13b5c224bf/Makefile#L1

https://github.com/amplify-edge/sys-share/blob/21c9d79819467c9439e346889d832d13b5c224bf/Makefile#L36

[alexadhy]

I still think best place to put it is in booty wiki (github).

Main just use a subset of booty, hence for users, it only documents small pieces of booty relevant to main

[gerardwebb]

ok go for what you think is best... you know booty and its design and intents better than me.