Implement the scoped agent-readiness subset for json.nlohmann.me:
- Add the mkdocs-llmstxt plugin to generate llms.txt from the nav
(full_output/llms-full.txt deliberately omitted to avoid dumping
500+ API reference pages into one giant file).
- Add a permissive robots.txt with a Sitemap reference.
- Add a build hook (hooks/copy_markdown_source.py) that copies each
page's Markdown source into the built site as a `<path>.md` sibling
of its HTML output, so agents/tools can fetch raw Markdown directly.
sitemap.xml was already emitted by default and needed no change.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>