docs: add an initial page about e2e testing

[mweibel]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Adds some initial docs about E2E testing which could be helpful for newcomers.

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

NONE

[k8s-ci-robot]

@mweibel: This issue is currently awaiting triage.

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[k8s-ci-robot]

Hi @mweibel. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[sedefsavas]

Wondering why there are changes in the index.md.
Not seeing in my local when I build the book.

[mweibel]

I'm not 100% sure but it could be an issue with how Makefile targets work. Because the CRD files didn't change the index.md doesn't need updating.
To double check I removed docs/book/src/crd/index.md and ran make -C docs/book build again. This produced the same output.
If you try to do this, do you still have no changes or do you get the same output?

[mweibel]

If you prefer, I can also remove this change from this PR to ensure no unrelated change is added with this PR. LMK if I should do this.

[sedefsavas]

No, after deleting and recreating I see these changes, so we can merge this.

[sedefsavas]

Thanks for adding this documentation @mweibel!

[sedefsavas]

/ok-to-test

[sedefsavas]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sedefsavas

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[pydctw]

/retest

[k8s-ci-robot]

@mweibel: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-aws-apidiff-main	9922a24	link	false	/test pull-cluster-api-provider-aws-apidiff-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.