Automate release process documentation

The release process today contains a large number of manual transformation steps.

Fixing this will make the release process significantly easier for release managers, as well as less error-prone.

Ideally the output formats (NEWS, CHANGES in source tree, and HTML snippets for http://couchdb.org/ website and http://blogs.apache.org/couchdb ) can be auto-generated from either the .rst files in share/doc/src using sphinx's .versionaddded/changed tags, or potentially from commit messages if this is appropriate.

CouchDB documentation is generated today from restructured text using python code, and rolled into the release documentation during `make distcheck`.