feat: Review & improve documentation before release

[lornakelly]

Description

Review, add and update the editor's documentation ahead of the MVP release. Cover all the READMEs, root and individual package READMEs. Also update storybook Welcome page.

Integration docs handled in a separate ticket #90

Motivation

As an NPM package meant to be host-agnostic and embeddable, the editor's documentation is the primary interface for adopters and new contributors - for a multi-vendor CNCF project, clear docs are what make external consumption and contribution actually viable rather than aspirational.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.