I ran into this issue as well, and after a little while, I found the cause (maybe you did too, but I think it would be good to share it here anyways):
The KB article of Sitecore https://kb.sitecore.net/articles/227897 referenced by the article on https://sitecore-community.github.io/docs/search/solr/Configuring-Solr-for-use-with-Sitecore-8/ says the following:
"Note: starting from Solr 6 default Config Sets are distributed without the schema.xml file. Schema.xml file should be created by duplicating and renaming the managed-schema file."
This is actually not correct! If you duplicate it and rename it, the original managed-schema file still exists and will be loaded first, instead of YOUR custom schema.xml. In other words, the custom Sitecore fields are not defined, hence the error you and I got when following the instructions :-). You need to replace or rename the file, or after duplicating, delete the original file; that will solve your issue.
Ps.: it is correct the the field definition isn't in your Solr config file, it now resides in the schema.xml file instead of the Sitecore config file (opposed to Lucene).