docs(docs): Add standardized architecture document structure

[stevenj]

Description

Adds arc42 architecture documents following the standard defined in catalyst-ci.
Also adds the first ADR to the repo, about arc42 and our ADR system itself.

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module