Create Concepts section

[abrennan89]

Partial work for #5054 + #5116

Proposed Changes

• Create new top level Concepts section
• Move Ducktyping docs
• Create snippets for serving and eventing explanations; content re-use

Preview links

• https://deploy-preview-5155--knative.netlify.app/docs/concepts/
• https://deploy-preview-5155--knative.netlify.app/docs/serving/
• https://deploy-preview-5155--knative.netlify.app/docs/eventing/
• https://deploy-preview-5155--knative.netlify.app/docs/reference/relnotes/ (page existed but was missing from nav.yml)

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abrennan89

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [abrennan89]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[netlify]

✅ Deploy Preview for knative ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	f1149d5
🔍 Latest deploy log	https://app.netlify.com/sites/knative/deploys/62e3df3a97bfae00096c295e
😎 Deploy Preview	https://deploy-preview-5155--knative.netlify.app/docs/eventing
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[psschwei]

/lgtm