feat: Add marketplace listing for plugins for website (#1369)

Adds marketplace.json which is the source of truth for
deephaven.io/plugins to feature offical and community created plugins.

Synced via s3.

---------

Co-authored-by: Don McKenzie <donmckenzie@illumon.com>

Author: dsmmcken

.github/workflows/marketplace.yml

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+# Validates the plugin marketplace on every PR that touches it, and syncs
+# marketplace/ to S3 via salmon on merge to main.
+name: Marketplace
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - 'marketplace/**'
+      - 'tools/validate-marketplace.mjs'
+      - '.github/workflows/marketplace.yml'
+  push:
+    branches:
+      - main
+    paths:
+      - 'marketplace/**'
+      - 'tools/validate-marketplace.mjs'
+      - '.github/workflows/marketplace.yml'
+
+jobs:
+  # Schema + link validation for marketplace.json.
+  marketplace-validate:
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: .nvmrc
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci --ignore-scripts
+
+      - name: Validate marketplace.json
+        run: npm run validate-marketplace
+
+  # Publish to S3 only after a marketplace change lands on main.
+  marketplace-sync:
+    needs: marketplace-validate
+    if: github.event_name == 'push'
+    runs-on: ubuntu-24.04
+    permissions:
+      id-token: write # Auth to AWS with OIDC
+      contents: read
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Sync marketplace to S3
+        uses: deephaven/salmon-sync@v1
+        with:
+          source: marketplace/
+          destination: deephaven/deephaven-plugins/marketplace/
+          production: true
+          temporary: false
+          aws-role: ${{ vars.DOCS_AWS_ROLE }}

marketplace/README.md

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+# Plugin marketplace
+
+`marketplace.json` is the source of truth for the plugin directory shown at [deephaven.io/plugins](https://deephaven.io/plugins). On merge to `main` this folder is synced to S3 and the website reads it from there, so a merged PR is all it takes to list, update, or remove a plugin.
+
+## Add your plugin
+
+1. Add an entry to the `plugins` array in [`marketplace.json`](./marketplace.json). The fields are documented in [`marketplace.schema.json`](./marketplace.schema.json); editors that understand `$schema` will autocomplete and validate as you type. The required fields are `name`, `description`, `author`, `repo`, and `image`:
+
+   ```json
+   {
+     "name": "My Plugin",
+     "description": "One sentence describing what it does, shown on the card.",
+     "author": "your-github-handle",
+     "repo": "https://github.com/your-github-handle/your-plugin",
+     "image": "/marketplace/images/my-plugin.png",
+     "registry": {
+       "kind": "pypi",
+       "package": "deephaven-plugin-my-plugin",
+       "url": "https://pypi.org/project/deephaven-plugin-my-plugin/"
+     },
+     "tags": ["python"]
+   }
+   ```
+
+2. Add a card image to [`images/`](./images) and reference it as `/marketplace/images/<file>.png`. It must be a PNG committed to that folder (external URLs are not allowed); keep it under 300 KB and roughly 16:9 (600×338 works well).
+
+3. Validate locally, then open a PR:
+
+   ```sh
+   npm run validate-marketplace
+   ```
+
+   CI runs the same check on every PR that touches this folder and blocks merge until it passes. Use `npm run validate-marketplace -- --skip-links` to skip the (slower) external link checks while iterating.
+
+## What the validator enforces
+
+Beyond the JSON schema, [`tools/validate-marketplace.mjs`](../tools/validate-marketplace.mjs) checks that:
+
+- plugin names are unique,
+- the `repo` owner matches the `author` (the author drives the official badge),
+- the `official` tag and `preInstalled` flag are reserved for Deephaven-owned plugins,
+- each plugin's `image` is a PNG that exists directly in `images/` and is under 300 KB,
+- no image in `images/` is orphaned (unreferenced by any plugin), and
+- every link resolves: `repo`, `href`, `registry.url`, and the author's GitHub profile.
+
+## Reserved fields
+
+`author: "deephaven"` plugins get the official badge automatically, and only they may set `preInstalled: true`. Don't add the `official` tag by hand; it is derived from the author.