Add ember-inflector in docs to fix docs page (cause ember-data bug)

[mkszepp]

The current ember-data version brakes docs page. We don't use any ember-inflector but ember-data brakes production build when there is not installed

This PR fix for the moment our docs build without downgrading ember-data (see emberjs/data#9501)

The error occurs only in production build, ember test or also with ember s there works fine

[runspired]

for any future wanderers, the bug here is that ember-inflector is installed as a dependency by ember-cli-mirage, but not included in production builds if ember-cli-mirage is the only thing that brings it as a dependency, which then breaks @embroider/macros.