Support rendering markdown articles in tutorials_src

Blog rewrite should also support rendering blogposts from pure markdown files. Should be easy by simply applying the logic used to transform markdown cells to the whole file.

Start by inspecting tutorials_src and the existing logic that transforms markdown cells. Trace the blog rewrite process for notebook-based articles, then determine how pure markdown files should enter that same path. Done means markdown blogposts in tutorials_src render successfully through the rewrite process.

Written by the indexing model from the issue text.