Setup docs

[chetanr25]

Add setup and project structure docs

Two new docs under docs/, plus a cleanup of CONTRIBUTING.md.

docs/ 1. SETUP.md - beginner setup guide. Prerequisites, make init / make fireform walkthrough,
verifying via the Swagger UI at localhost:8000/docs, day-to-day make commands, and a troubleshooting
section for the usual failures (Docker not running, port 8000 taken, slow model pull). Also points to the
frontend repo since it lives separately now.

docs/ 2. PROJECT_STRUCTURE.md - explains how the repo is laid out and where new code should go. Covers
the layering inside app/ (routes → services → repositories → db) with concrete recipes: how to add an
endpoint, a table, an env var, a custom error. Should save reviewers from repeating the same "this
belongs in services" comments on every PR.

CONTRIBUTING.md changes:

• Setup instructions were duplicated between CONTRIBUTING and the new guide, so the Local Development
  Setup section is now just a link to it. The old desktop app section referenced fireform-frontend, which
  doesn't exist in this repo anymore removed.
• Added the Discord invite and a note on how issues work here: we don't formally assign them, anyone is
  free to pick one up and raise a PR. First PR that actually fixes the issue generally gets preference, so
  coordinate on Discord to avoid duplicate work.

Fixes: #519
Part of #512