WIP generate markdown docs from eep48 docs

[uwiger]

The idea:

EDoc supports generation of doc chunks from edoc syntax, using e.g. a config like this:

{edoc_opts,
  [{doclet, edoc_doclet_markdown},
   {preprocess, true},
   {layout, edoc_layout_chunks}]}.

Running rebar3 edoc with this config once, will insert -moduledoc chunks into the source corresponding to the old edoc comments. It would be nice to be able to delete the old edoc after this, and still generate the same kind of documentation.

This PR (WIP) checks if there is a doc chunk for a module, and extracts descriptions from it if legacy edoc descriptions are missing (if they're not missing, they are used instead).

Only tested so far, with some success, for module-level and function-level @doc tags.