Remove release archives from master

These archives made it difficult to find things in the GitHub interface,
and take up a lot of space in a checked-out repo for something that is
not useful to most people checking out the repository.

The main purpose of these archives is to make old versions and
documentation available on the Sinon website that is run out of this
repo. This can be supported by using a separate branch for website
releases, and to maintain the archives.

Following this commit, the `npm version` scripts will be updated to
automatically handle archiving the releases in the new releases branch
and keeping it up to date with master.

Also remove the directories we removed from .prettierignore, since they
don't exist any more.

Author: cincodenada

.prettierignore

@@ -5,8 +5,6 @@ tmp/
 .sass-cache
 docs/_site/
 docs/js/
-docs/releases/
-docs/_releases/
 docs/assets/js/
 CHANGELOG.md
 docs/changelog.md