📡 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/requirements.txt

@@ -7,5 +7,6 @@ mkdocs-material-extensions==1.3.1                   # extensions
 mkdocs-minify-plugin==0.8.0                         # plugin "minify"
 mkdocs-redirects==1.2.3                             # plugin "redirects"
 mkdocs-htmlproofer-plugin==1.5.0                    # plugin "htmlproofer"
+mkdocs-llmstxt==0.5.0                               # plugin "llmstxt"
 
 PyYAML==6.0.3                                       # linter