404 for HIG docs - redirect old locations while updating major links

## What Happened

Hitting 404s for HIG docs from prominent places (including first-party pages).

## Expected Behavior

Get redirected from `https://elementary.io/docs/human-interface-guidelines` to `https://docs.elementary.io/hig` so the re-organization of pages doesn't matter to most anyone (it takes a long time to clean up old links).

## Steps to Reproduce

See any of these places linking to HIG docs, which then 404:

1. "HIG" linked in https://blog.elementary.io/developer-tips-backgrounding-system-integration (found on https://islinuxabout.xyz/systray/)
1. huge "design" button centered on splash page of https://developer.elementary.io (found atop the elementary site)
1. [web results for `elementary os hig`](https://duckduckgo.com/?q=elementary+os+hig) surface `https://elementary.io/docs/human-interface-guidelines` as the top result

## Platform Information

web