Clean up Hugo deprecation warnings.

[evankanderson]

Fixes the following warnings, updates to the latest version of the Docsy theme

WARN 2020/08/04 14:26:46 Page.Hugo is deprecated and will be removed in a future release. Use the global hugo function.
WARN 2020/08/04 14:26:46 Page.Dir is deprecated and will be removed in a future release. Use .File.Dir

  Replace Autoprefixer browsers option to Browserslist config.
  Use browserslist key in package.json or .browserslistrc file.

  Using browsers option cause some error. Browserslist config 
  can be used for Babel, Autoprefixer, postcss-normalize and other tools.

  If you really need to use option, rename it to overrideBrowserslist.

  Learn more at:
  https://github.com/browserslist/browserslist#readme
  https://twitter.com/browserslist

[evankanderson]

Staged sample: https://5f29e5a2c46b2900081b06d7--knative.netlify.app/

Deploy log without errors: https://app.netlify.com/sites/knative/deploys/5f29e5a2c46b2900081b06d7

[RichieEscarez]

FYI, part of the reason nobody attempted to update the Docsy version was that we were on one of the very first alpha-level versions and a lot has changed since then. I will spend some time clicking through most pages and running link checking randomly to make sure none of our Knative customizations have broken. Thanks for this and #200 to get us up-to-date!

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: evankanderson, RichieEscarez

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