Add v1.18.0, drop v1.4.0

[philrz]

New version generated by make version REF=v1.18.0.

Also, in the spirit of what I started in #82, I also followed these instructions to drop the oldest version at the same time to keep the site from getting so huge that the link checker would fail. Here's a summary of the steps for quick reference next time:

# Edited "versions.json" and deleted the "v1.4.0" entry
git rm -r versioned_docs/version-v1.4.0
git rm versioned_sidebars/version-v1.4.0-sidebars.json