Also add example configuration for documentation linters.

The existing config did not work, it gave an error.
This new config makes it use our existing yarn task.

This is because when multiple versions of Bundler are installed (e.g.
1.17.3 and 2.1.4), Bundler will choose the 2.x version based on the
`gems.rb` / `gems.locked` file names but it chooses the 1.17.x version
if the we use the legacy `Gemfile` / `Gemfile.lock` file names.

For now, we'll use the legacy file names to be consistent with the main
`gitlab`'s `Gemfile.lock` and to avoid unexpected changes in the
working tree.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Running Overcommit outside of the context of a Gemfile can lead to
conflicts when requiring gems. Using a dedicated Gemfile instead of the
default one avoids loading all the app's gems before running Overcommit.

Signed-off-by: Rémy Coutable <remy@rymai.me>

- it would be nice to have a working file in here as an
  example file.
- this brings us one step closer to hopefully utilizing
  overcommit in CI and/or at least on each dev's machine
- next step would be for gdk install to copy it over,
  drop the extension and install it
- or drop the .example on this file and make it the default,
  and users will merely have to run a `overcommit -i` after
  installing overcommit gem