docs: upstream_block ADR

[kdmccormick]

WIP

[ormsbee]

For anyone watching this ADR, @kdmccormick is doing some experimentation in https://github.com/openedx/edx-platform/pull/34925 to help inform this ADR.

The current thinking is confine it purely to a Studio concept and continue to write to the normal field name so that LMS runtime code does not need to be changed (and we don't have to deal with complexities around inheritance). It would look something like this:

<problem
   display_name="course override title"
   max_attempts="2"
   show_answer="true"

   upstream_block="lb:..."
   upstream_version="15"
   upstream_overridden="display_name,max_attempts"

   upstream.display_name="lib default title"
   upstream.max_attempts="2"
   upstream.show_answer="true"
>  ... content... </problem>

so, in this case, we know that max_attempts is explicitly overridden to 2, even though that's also the upstream default; on the contrary, show_answer is whatever the library default is.

However, all of this is still in flux, so if you want to follow the latest, please join the #content-libraries-relaunch-dev Slack room.

[kdmccormick]

Closed in favor of https://github.com/openedx/edx-platform/pull/35421