📡 make documentation more LLM friendly (#5244)

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>

docs/mkdocs/hooks/copy_markdown_source.py

@@ -0,0 +1,25 @@
+"""Copy each documentation page's Markdown source into the built site."""
+
+# Creates a `<path>.md` sibling of each HTML output (for example,
+# `features/comments/` becomes `features/comments.md`) so agents and tools can
+# fetch the raw Markdown directly instead of parsing rendered HTML.
+
+import os
+import shutil
+
+_pages = []
+
+
+def on_files(files, config):
+    global _pages
+    _pages = [f for f in files if f.is_documentation_page()]
+    return files
+
+
+def on_post_build(config):
+    site_dir = config["site_dir"]
+    for file in _pages:
+        url = file.url.rstrip("/")
+        target = os.path.join(site_dir, (url or "index") + ".md")
+        os.makedirs(os.path.dirname(target), exist_ok=True)
+        shutil.copyfile(file.abs_src_path, target)