docs: setup new SDK documentation for 2.0 and change sdk build version to 2.0...

move upgrading doc into a new guide for "updating" with
the new manager tools and remove the previous updating doc
from the dev guide. set up preliminary download page and guide
for installing the full SDK. add the requirements doc in sdk/
for all general purposes. revise the download 'thank you'
message.

bug: 2160782