Clarify or remove stale ChartsV2 architecture doc

[vishxrad]

Summary

packages/react-ui/src/components/ChartsV2 currently contains only ARCHITECTURE.md. The document describes implementation details, tests, and utilities for ChartsV2, but there is no implementation in that directory. This can confuse contributors trying to understand the current chart code.

Suggested implementation

Decide whether this document is still useful as a proposal/historical note. Then either:

• move it to a more appropriate docs/proposals location,
• update the title and contents to clearly mark it as historical/proposed architecture,
• or remove it if it is obsolete.

Acceptance criteria

• Contributors can tell whether ChartsV2 is current, planned, or obsolete.
• Any stale statements about existing files/tests are corrected or removed.
• The current chart implementation remains documented where appropriate.