docs(image-reflector-controller/docs/spec/v1beta1/imagerepositories.md): elaborate on how to enable autologin for ECR

[lloydchang]

fluxcd/flux#1619
fluxcd/flux2#107
fluxcd#11
fluxcd#139
fluxcd#147
fluxcd#174
fluxcd#179
fluxcd#180
fluxcd/website#601
fluxcd#193
fluxcd/website#611
https://fluxcd.io/docs/guides/image-update/#aws-elastic-container-registry

Signed-off-by: lloydchang lloydchang@gmail.com

[lloydchang]

• Inline directly relevant quotes so that people don’t have to click through the links just to better understand the prerequisite
• Link many related in one place (Git commit and PR description) for broader context
• Fix a few grammar and typo issues in other parts of document

Thank you @kingdonb

[kingdonb]

Thanks for the review and the additions. I rebased this on my own PR branch, in order to be sure the example patch and the clarifying statement that explicitly mentions image-reflector-controller are both still included.

I also felt like there were too many redundant links in your patch, so I removed most of them. I think I have still included all of the important references that you meant to add.

I'll include these in my PR, thanks again @lloydchang ! 🎉