Update documentation open-scd

[noraeb]

Description

Update the README files for com-pas/open-scd and com-pas/compas-architecture to reflect the current relationship between com-pas/open-scd and com-pas/compas-open-scd.

Background

Historically, com-pas/compas-open-scd was a fork of openscd/open-scd. This is no longer the case (only technically). As part of a recent restructuring:

• All of open-scd's OpenSCD-specific packages (xml, openscd, plugins, core) were removed from compas-open-scd.
• compas-open-scd now only retains its own CoMPAS-specific packages (compas and external-packages).
• open-scd continues to maintain and develop the OpenSCD-specific code, which is now published to npm.
• compas-open-scd consumes those OpenSCD packages as npm dependencies rather than as part of a shared/forked codebase.
• open-scd still maintains its own distribution package for local development purposes (independent of the npm-published packages).

This change is not currently reflected in the documentation, which may confuse contributors, maintainers, or new team members about how the two repositories relate to each other and how releases flow between them.

Goal
Documentation across open-scd and compas-architecture should clearly explain:

• What each repository is responsible for.
• That compas-open-scd is no longer a fork of open-scd.
• How code flows from open-scd → npm → compas-open-scd.
• How releases/versioning work for both repos.

[Sander3003]

I suggest to add the use of the Openscd API/theming to be added to the compas-architecture documentation. https://github.com/openscd/oscd-api/tree/main/docs. If this is a big scope increase, a new issue shoud be created.