fix: add redirect for removed machine learning documentation

[jbatesuk]

Summary

Adds an nginx rewrite rule to redirect all URLs under /docs/machine-learning/ to /docs.

Background

The machine learning documentation has been removed. Without a redirect, users visiting any /docs/machine-learning/* URL will get a 404. This adds a permanent redirect to send them to the docs index instead.

Changes

• nginx.conf: Added rewrite ^/docs/machine-learning(/.*)?$ /docs permanent;