docs: add contributing section (#3384)

Adds a Contributing section to the Fresh documentation covering:

- Repository structure and package layout
- Getting started (clone, `deno task ok`)
- Development workflow (docs site, vite demo, `links` for external
projects)
- Testing (commands, conventions, known local issues)

Single page at `docs/latest/contributing/index.md`.

---------

Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

docs/latest/contributing/index.md

@@ -0,0 +1,93 @@
+---
+description: |
+  Guidelines and resources for contributing to Fresh.
+---
+
+# Contributing
+
+Fresh is organized as a monorepo with multiple packages:
+
+- **`packages/fresh/`** - Core Fresh framework (`@fresh/core`)
+- **`packages/plugin-vite/`** - Vite integration plugin (`@fresh/plugin-vite`)
+- **`packages/plugin-tailwindcss/`** - Tailwind CSS v4 plugin
+- **`packages/plugin-tailwindcss-v3/`** - Tailwind CSS v3 plugin
+- **`packages/init/`** - Project scaffolding tool
+- **`packages/update/`** - Fresh update utilities
+- **`www/`** - Documentation website (fresh.deno.dev)
+
+## Getting Started
+
+Prerequisites: [Deno](https://deno.com/) (latest version) and Git.
+
+```sh Terminal
+git clone https://github.com/denoland/fresh.git
+cd fresh
+deno task ok
+```
+
+`deno task ok` runs formatting, linting, type checking, and the full test suite.
+Run it before submitting any pull request.
+
+The repository uses Deno
+[workspaces](https://docs.deno.com/runtime/fundamentals/workspaces/) so all
+packages in `packages/` are automatically available to each other using their
+published names (`@fresh/core`, `@fresh/plugin-vite`, etc.).
+
+## Development
+
+The `www/` directory and the Vite plugin demo both use local Fresh packages,
+making them good integration tests:
+
+```sh Terminal
+deno task www           # docs site dev server
+deno task build-www     # docs site production build
+
+deno task demo          # vite demo dev server
+deno task demo:build    # vite demo production build
+deno task demo:start    # serve vite demo production build
+```
+
+### Testing in External Projects
+
+To use your local Fresh checkout in a separate project, add
+[`links`](https://docs.deno.com/runtime/fundamentals/configuration/#links) to
+the project's `deno.json`:
+
+```json deno.json
+{
+  "imports": {
+    "@fresh/core": "jsr:@fresh/core@^2.0.0",
+    "@fresh/plugin-vite": "jsr:@fresh/plugin-vite@^1.0.0"
+  },
+  "links": [
+    "../path/to/fresh/packages/fresh",
+    "../path/to/fresh/packages/plugin-vite"
+  ]
+}
+```
+
+This overrides the JSR packages with your local versions. Changes are reflected
+immediately without rebuilding.
+
+## Testing
+
+```sh Terminal
+# All tests (parallel)
+deno task test
+
+# A specific test file
+deno test -A packages/fresh/src/app_test.ts
+
+# Filter by test name
+deno test -A --filter "test name pattern"
+
+# Update snapshots after intentional output changes
+deno test -A --update-snapshots path/to/test.ts
+```
+
+Tests use `@std/expect` for assertions, follow the `*_test.ts` naming
+convention, and require the `-A` flag. Snapshot tests are stored in
+`__snapshots__/` directories.
+
+Some tests may fail locally but pass in CI (`Could not find server address`,
+`Text file busy (os error 26)`) — these can be safely ignored.

docs/toc.ts

@@ -103,6 +103,10 @@ const toc: RawTableOfContents = {
           ["session-management", "Session management", "link:latest"],
         ],
       },
+      contributing: {
+        title: "Contributing",
+        link: "latest",
+      },
     },
   },
   "1.x": {