add step for installing sugar for e2e

[vaikas]

Since E2E tests rely for some tests on sugar controller, we should document it in our development. One of our users ran into this.

Proposed Changes

• Document that sugar is opttional component, but if you're running e2e tests you need to install it.

Release Note

Docs

[mattmoor]

Produced via:
prettier --write --prose-wrap=always $(find -name '*.md' | grep -v vendor | grep -v .github | grep -v docs/cmd/)

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-eventing-integration-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-knative-eventing-integration-tests:

test/e2e.TestBrokerChannelFlowTriggerV1BrokerV1
test/e2e.TestBrokerChannelFlowTriggerV1BrokerV1/Channel-messaging.knative.dev/v1beta1

[vaikas]

    creation.go:249: Failed to create v1 trigger "e2e-brokerchannel-trigger2": conversion webhook for eventing.knative.dev/v1beta1, Kind=Trigger failed: Post https://eventing-webhook.knative-eventing-unqhto62h6.svc:443/resource-conversion?timeout=30s: EOF

[pierDipi]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pierDipi, vaikas

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