docs: add an experimental elua based documentation generator

This is an Eolian-using doc gen written with Elua. It will be used to generate
the Dokuwiki skeleton for new EFL docs.

@feature

src/scripts/elua/apps/docgen/README.md

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+# docgen
+
+This is the upcoming documentation generator for EFL. It takes documentation
+described in Eo files and turns it into a DokuWiki structure (with possibility
+of adapting it to other systems later on).
+
+Use the supplied build.sh script to generate. By default, it assumes that a
+directory called "dokuwiki" is present in the current directory, so symlink
+your dokuwiki setup into it (or change the path).