Update documentation links in examples and website pages

[aLekSer]

Update links to 1.15 kubernetes version.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug
/kind cleanup

/kind documentation

/kind feature
/kind hotfix

What this PR does / Why we need it:

Which issue(s) this PR fixes:

Work on #1478

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 3a78c0f0-4fe6-4947-8cf7-c0f238a5641c

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.6.0-10f7e9d
• image: gcr.io/agones-images/agones-sdk:1.6.0-10f7e9d
• image: gcr.io/agones-images/agones-ping:1.6.0-10f7e9d
• Linux C++ SDK (build): agonessdk-1.6.0-10f7e9d-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.6.0-10f7e9d.zip

A preview of the website (the last 30 builds are retained):

• https://10f7e9d-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1554/head:pr_1554 && git checkout pr_1554
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.6.0-10f7e9d

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aLekSer, pooneh-m

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 [pooneh-m]

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

[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Failed 😱

Build Id: 89ae6b06-bc55-420a-9fde-91e6f18f1220

• Cloud Build view
• Cloud Build log download

To get permission to view the Cloud Build view, join the agones-discuss Google Group.

[agones-bot]

Build Succeeded 👏

Build Id: 0461f2e9-3945-4b8a-98f9-cb8ba3994d16

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.6.0-6bbbf09
• image: gcr.io/agones-images/agones-sdk:1.6.0-6bbbf09
• image: gcr.io/agones-images/agones-ping:1.6.0-6bbbf09
• Linux C++ SDK (build): agonessdk-1.6.0-6bbbf09-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.6.0-6bbbf09.zip

A preview of the website (the last 30 builds are retained):

• https://6bbbf09-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1554/head:pr_1554 && git checkout pr_1554
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.6.0-6bbbf09