docs: v0.1.0

[krakenhavoc]

This pull request introduces comprehensive documentation and onboarding improvements to the KrakenKey app repository. It adds a new top-level README.md that details the architecture, setup instructions, and related resources for the project, and it significantly expands backend documentation, especially around billing, subscriptions, and certificate issuance flows. Additionally, it updates the backend's own README to reflect new API modules and improves documentation references.

Major documentation additions and improvements:

General project documentation:

• Added a new top-level README.md describing the overall architecture (backend, frontend, shared types), setup instructions (both devcontainer and manual), environment variables, scripts for backend/frontend, API docs, and related repositories.

Backend module and API documentation:

• Updated backend/README.md to list new API modules (billing, organizations, endpoints, health, metrics, notifications, feedback, throttler) and improved documentation links/descriptions for easier navigation.

Billing and subscription documentation:

• Added backend/docs/BILLING.md with detailed explanations of plan tiers, resource limits, Stripe integration, API endpoints for billing actions, subscription lifecycle, organization billing, and the subscription entity schema.

Certificate issuance flow documentation:

• Added backend/docs/CERTIFICATE_FLOW.md covering the end-to-end process for certificate requests, including CSR generation, domain verification, ACME issuance, auto-renewal, revocation, retrying failed certificates, and plan-based quotas.