[redirects] Don't version control files under the auto-generated public directory

[chalin]

Currently the Netlify _redirects file is being edited directly from the Hugo-generated public directory. It should not be setup in that way.

Instead move the file to static/_redirects.

Later we can consider switching to using Hugo aliases, and eventually an implementation of Hugo aliases via Netlify redirects as has been done elsewhere, such as open-telemetry/opentelemetry.io#781.

Waiting for the following to be merged first:

• Add redirect for release predicate #23
• Add missing predicate spec redirects, update Statement v1 redirect #38

[lukpueh]

@chalin, this should be unblocked now