Add info.rkt for the parsack collection.

For scribblings. See issue #42. Although I think adding this is
necessary, I'm not sure it's sufficient: Although I do now see
`raco setup parsack` building parsack.scrbl, and an XREPL
`,doc parse-result` works, I don't see it using Search in the web
browser. So either something else remains to be done, or, it was
sufficient and I just have a stale cache.
Merge pull request #43 from greghendershott/collection-info-rkt

Add info.rkt for the parsack collection.
fix broken links in local docs; fixes #42

- Note:

With these changes, compiling docs for the web with the magic incantation:

  scribble --html
           +m
           --redirect-main http://docs.racket-lang.org
           --dest .
           parsack.scrbl

now produces the warning:

  "some cross references may be broken due to undefined tags"

Although all the links in the resulting html appear correct.