Add custom catalog example. (#100)

Author: cmrigney

examples/custom-catalog/README.md

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+# Using the MCP Gateway with a Custom Catalog
+
+This example shows how you can use your own custom catalog in the MCP Gateway:
+
++ Defines a custom catalog `catalog.yaml` from which servers can be chosen from.
++ Picks the server `duckduckgo` from the custom catalog.
++ Uses SSE for the transport and can be connected to via `http://localhost:8811/sse`.
+
+## How to run with compose
+
+```console
+docker compose up
+```
+
+Add client services, like Agents, that connect with the `sse` protocol on port `8811`.
+
+## Running without Compose
+
+```console
+docker mcp gateway run --catalog=./catalog.yaml --servers=duckduckgo --transport=sse --port=8811
+```

examples/custom-catalog/catalog.yaml

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+registry:
+  duckduckgo:
+    description: A Model Context Protocol (MCP) server that provides web search capabilities through DuckDuckGo, with additional features for content fetching and parsing.
+    title: DuckDuckGo
+    type: server
+    image: mcp/duckduckgo@sha256:68eb20db6109f5c312a695fc5ec3386ad15d93ffb765a0b4eb1baf4328dec14f
+    allowHosts:
+      - html.duckduckgo.com:443

examples/custom-catalog/compose.yaml

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+services:
+  gateway:
+    image: docker/mcp-gateway
+    ports:
+      - "8811:8811"
+    command:
+      - --servers=duckduckgo
+      - --catalog=/mcp/catalog.yaml
+      - --transport=sse
+      - --port=8811
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+      - ./catalog.yaml:/mcp/catalog.yaml