Import couchapp org

A first cut at documentation copied from the last snapshot of couchapp.org on web.archive.org, taken in Feb 2014.  couchapp.org is currently still offline.

The pages have been converted from HTML back into markdown, and tidied up. The commit comments describe the details.

There is a list of broken links, and a Perl script to check the links.  I infer some pages are missing.

The result needs to be copied into /doc/ somewhere, possibly in a subdirectory, however I'm leaving that for upstream to do.