"stable" docs point to 0.18.0

[mattwthompson]

Describe the bug

I think we have the background to understand the context from only an image (recalling releases 0.18.1 and 0.19.0 exist):

To Reproduce

Go to https://docs.openforcefield.org/projects/toolkit/en/stable/releasehistory.html

Output

See above

Computing environment (please complete the following information):

• Operating system
• Output of running conda list

Additional context

[j-wags]

The problem is we were using an RTD image for docs building that must have become deprecated during the release process 🤦

This will need a new release to resolve