Motivation and context:
We had lots of files in our docs with underscores, which results in URLs with underscores. The internet seems to have standardized on hyphens between words, so who are we to not conform?

This PR uses the new redirects extension in Nengo Sphinx Theme to redirect all old URLs to the new ones. The minimum version has been updated accordingly.

How has this been tested?
Verified that all the redirect pages are being generated, and take you to the right place.

How long should this take to review?

• Average (neither quick nor lengthy)

Types of changes:

• Non-code change (touches things like tests, documentation, build scripts)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• [NA] I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.