Rename and reword auto-tls feature to external-domain-tls

[ReToCode]

Partially knative/serving#14368

Proposed Changes

• Rename auto-tls to external-domain-tls
• Reword the existing pages to prepare for the future flags we introduce
  • Make it more clear that the current feature is only for external domains
  • Point to our existing work to bring HTTPS to cluster-local domains as well
• Remove duplications about installing cert-manager
• Reorder the navigation to have a new section for encryption. We'll add an overview page to that section to explain the three different parts of Serving Encryption in the future
• Rename pages to better reflect what they are trying to explain

Review links for changed pages:

• https://deploy-preview-5692--knative.netlify.app/docs/install/installing-cert-manager/
• https://deploy-preview-5692--knative.netlify.app/docs/install/yaml-install/serving/install-serving-with-yaml/
• https://deploy-preview-5692--knative.netlify.app/docs/serving/encryption/using-certificates-in-networking-layer/
• https://deploy-preview-5692--knative.netlify.app/docs/serving/encryption/enabling-automatic-tls-certificate-provisioning/
• https://deploy-preview-5692--knative.netlify.app/docs/serving/services/custom-tls-certificate-domain-mapping/
• https://deploy-preview-5692--knative.netlify.app/docs/serving/services/http-protocol/

/hold - work in networking/serving is not merged yet

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ReToCode

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 [ReToCode]

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	8c17e9f
🔍 Latest deploy log	https://app.netlify.com/sites/knative/deploys/650d2d80b99eb6000804cabe
😎 Deploy Preview	https://deploy-preview-5692--knative.netlify.app/docs/serving
📱 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 configuration.

[ReToCode]

Depends on this merged: knative/networking#858

[ReToCode]

/unhold as networking PR is merged

/assign @nak3
/assign @dprotaso
/assign @skonto

[ReToCode]

/unhold

[nak3]

/lgtm