feat: route plain-text and Quarto/R Markdown files to the Markdown backend (#3161)

* feat: route plain-text and Quarto/R Markdown files to the Markdown backend

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

* updated the README and index.md

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

* _mime_from_extension: Added a check for extensions in the intersection of XML_USPTO and MD extension lists (currently just txt). These ambiguous
  extensions get pass — leaving mime=None — so the full content-probing chain (_detect_html_xhtml → _detect_csv → text/plain fallback) runs instead of
   prematurely assigning text/markdown.

  _guess_from_content: Removed the elif InputFormat.MD in formats MD fallback for text/plain content. Unrecognised .txt content now correctly returns
  None. MD is only returned from explicit mime types (text/markdown, text/x-markdown) which come from unambiguous extensions like .md, .text, .qmd,
  .rmd.

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

* ran pre-commit

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

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Signed-off-by: Peter Staar <taa@zurich.ibm.com>