Docs: Add markdown content filter example for REST API

[per1970]

Summary

Adds a Markdown with Content Filter (Pruning / BM25) example to the Docker server README REST API section (deploy/docker/README.md), between the Simple Crawl and Streaming examples.

What it covers:

• The working payload shape for content_filter, nested inside markdown_generator with the {"type": "ClassName", "params": {...}} serialization
• The silent-drop pitfall: a top-level content_filter key in crawler_config is ignored by the untrusted config loader (HTTP 200, empty fit_markdown) — this cost real debugging time, and the failure mode is invisible unless you check fit_markdown
• Response fields: fit_markdown / fit_html vs the always-present raw_markdown
• BM25 variant (user_query) and a pointer to docs/examples/docker/demo_docker_api.py (Demo 2a–2c) for more configurations

Verification

Payload tested live against the 0.9.2 Docker server:

• https://httpbin.org/html: raw 3598 → fit 3598 chars (pruning, threshold 0.45 fixed)
• https://www.bbc.com/news: raw ~22K → fit ~9.9K chars (nav/cookie boilerplate stripped)

Transparency: Prepared with assistance from my Hermes AI agent (Muninn), based on live verification against the 0.9.2 server. The example payload in the PR is the exact payload that was tested. Happy to adjust wording or add the same note to demo_docker_api.py if preferred.