Add Httpstan Compatibility Chart to Documentation

[AlexanderNenninger]

Describe the problem with the documentation

Currently, the latest httpstan release is 4.11.0 while the latest supported httpstan version is 4.10.1. This is not apparent in the instructions for installing pystan from source.

Suggest a potential alternative/fix

Add specific httpstan version requirements to documentation.

[riddell-stan]

Good point. The normal release process got delayed a bit. This should be resolved soon.