Add `manuals#show` routes
Add `manuals#show` routes

This commit updates the `routes.rb` to enable `manuals#show` pages using
the `content_id` as the `param`.

alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher