Updates to the tutorial docs

[hickeyma]

Description of the change:
While using the operator-sdk docs, I encountered the following issues:

• Link in https://sdk.operatorframework.io/docs/building-operators/golang/tutorial/#prerequisites for install is incorrect. It should be https://sdk.operatorframework.io/docs/building-operators/golang/installation/
• Error when run: make docker-build IMG=hub.docker.com/$USERNAME/memcached-operator:v0.0.1 in https://sdk.operatorframework.io/docs/building-operators/golang/tutorial/#build-and-push-the-image:

[...]
Failure [0.005 seconds]
[BeforeSuite] BeforeSuite 
/home/marty/projects/memcached-operator/controllers/suite_test.go:52

  Unexpected error:
      <*fmt.wrapError | 0xc0003d0ac0>: {
          msg: "failed to start the controlplane. retried 5 times: fork/exec /usr/local/kubebuilder/bin/etcd: no such file or directory",
          err: {
              Op: "fork/exec",
              Path: "/usr/local/kubebuilder/bin/etcd",
              Err: 0x2,
          },
      }
      failed to start the controlplane. retried 5 times: fork/exec /usr/local/kubebuilder/bin/etcd: no such file or directory
  occurred

  /home/marty/projects/memcached-operator/controllers/suite_test.go:62

Installing kubebuilder fixes the issue. Ref: Issue #4203.

• In https://sdk.operatorframework.io/docs/building-operators/golang/tutorial/#create-a-memcached-cr:

kubectl get deployment command should not display memcached-operator-controller-manager as it is not in the default namespace

Motivation for the change:
Improve the docs.

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[hickeyma]

Thanks for the review and comments @estroz. I replied inline to your feedback.

[hickeyma]

Updates and feedback provided inline. Thanks for review @camilamacedo86 @estroz.

[hickeyma]

@camilamacedo86 @estroz Found the issue with the test suite and responded in #4395 (comment). Just wondering if a note should be added until a fix becomes available?

[hickeyma]

Updated following latest review comments from @estroz. It is good for review again @camilamacedo86 @estroz.

[estroz]

/lgtm

[hickeyma]

Thanks for the reviews @camilamacedo86 and @estroz and the merge @estroz.

[estroz]

@hickeyma thanks a ton for the contribution!