Add ADR for build pipeline consolidation

[jjroelofs]

Linked issues

• Consolidate Grunt + Webpack into a single build pipeline #775

Solution

Add an Architecture Decision Record (ADR) proposing the consolidation of the dual Grunt + Webpack build pipeline into a single Webpack-based pipeline. This PR is for team discussion and decision-making before implementation begins.

The ADR documents:

• Current dual-system architecture and its problems
• Proposed Webpack-only approach
• Expected benefits (single config, tree-shaking, fewer dependencies)
• Risks and migration concerns
• Alternatives considered (Vite, keeping Grunt)

Checklist

• I have read the CONTRIBUTING.md document.
• My commit messages follow the contributing standards and style of this project.
• My code follows the coding standards and style of this project.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Need to run update.php after code changes
• Requires a change to end-user documentation.
• Requires a change to developer documentation.
• Requires a change to QA tests.
• Requires a new QA test.
• I have updated the documentation accordingly.
• All new and existing tests passed.