docs: add system architecture document with C4 diagrams

[Agencybuilds]

Closes #191
Overview
This PR addresses the lack of high-level architectural documentation in the repository by introducing a comprehensive docs/ARCHITECTURE.md file. It provides new contributors and developers with a clear overview of how the VertexChain components interface with each other and the underlying technologies (Next.js, NestJS, PostGIS, IPFS, and Stellar/Soroban).

Changes Made

• Created docs/ARCHITECTURE.md containing:
  • C4 Level 1 (System Context): Displays the relationship between End Users, the VertexChain platform, and external systems (Stellar Blockchain, IPFS/Pinata).
  • C4 Level 2 (Containers): Shows the technology stack breakdown (Next.js Frontend, NestJS Backend, PostgreSQL + PostGIS Database, Pinata API, and Stellar/Soroban RPC).
  • C4 Level 3 (Components): Decomposes the internal modules of the NestJS Backend (Controllers, Services, Repositories, Caching, and Indexer) and the Rust-based Soroban Smart Contracts (GistRegistry, Multisig, Governance, and BatchWallet).
  • Key Data Flows: Includes sequential flow diagrams for Gist Creation and On-Chain Event Indexing.

Verification

• Verified Mermaid syntax compatibility for seamless GitHub rendering.
• Verified that all internal document anchor links navigate correctly.

[snowrugar-beep]

Heads-up: CI was green a moment ago but this PR is no longer marked MERGEABLE, so I didn't auto-merge it. Could you rebase against main and re-push? \xf0\x9f\x99\x8c

[snowrugar-beep]

Hey \xf0\x9f\x91\x8b \xe2\x80\x94 small update: my very first note on this PR suggested the mergeable status was a transient hiccup. After more careful checking, it turns out the branch now actually has merge conflicts against main (likely because other PRs landed in between). Could you rebase against the latest main and re-push the PR? Once that's clean we can merge right away. Thanks, and apologies for the confusion on the earlier message!

[snowrugar-beep]

LGTM