This extension http://pgxn.org/dist/italian_fts/ has a document (LEGGIMI.rst) not included in the docs because the format is not recognized.
I understand there is already a perl parser for it. If there wasn't though it would be easy to wrap the rst2html command you get installing the "docutils" Python package.
Note that reST is an extensible markup, with extension points called "directive" (more or less a "div") and "role" (more or less a "span"). A reST docuent may require extensions to be rendered properly (this is the case of http://pgxn.org/dist/pgmp/). GitHub renderer for instance is broken, and the unknown directives are omitted from the output instead of being rendered unparsed (see https://github.com/dvarrazzo/pgmp/blob/master/docs/mpz.rst compared with raw). In order to prevent this problem and to obtain some docs, for pgmp 1.0b3 we have decided to include the html in the package. I think the case of an extension containing both the original .rst document and the rendered html may be non uncommon: in this case probably pgxn should display the html only.
FYI, I've already created a ticket for this issue over here. The Perl reST parser is awful, though. I suspect someone other than me will need to make it work.