Allow configuration of govuk-frontend package being used

[romaricpascal]

What should change

Provide a way for projects using tech_docs_gem to override where govuk-frontend gets loaded from so it's not tied to the version set by tech_docs_gem.

A quick path for that may be to provide a govuk_frontend_path option to GovukTechDocs.configure letting people point to the package in their own node_modules.

User need

During our latest release of GOV.UK Frontend, we noticed that our tech docs were not picking up the style fixes brought by our package.

Following the trails, this looked due to the tech_docs_gem configuring Sprockets to load its own version of govuk-frontend which it would be great to have some control on or be able to override.