Adds instructions for customers to complete their AWS annual subscription registration

[kwabena-ansah]

See original PR here: https://github.com/elastic/cloud/pull/139303
Preview is here: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/990/deploy-manage/deploy/elastic-cloud/complete-registration-of-an-annual-subscription-on-aws-marketplace

I'm not sure if this should be marked as hidden but I did it to follow similar marketplace docs

[florent-leborgne]

Thanks @kwabena-ansah!

I see the page is hidden here, but not in the asciidoc counterpart, which one should it be?

[kwabena-ansah]

I see the page is hidden here, but not in the asciidoc counterpart, which one should it be?

Yea I don't think it should be hidden but I did it for consistency with the others. Should I unhide all of these as well ?

- hidden: deploy/elastic-cloud/create-monthly-pay-as-you-go-subscription-on-aws-marketplace.md
- hidden: deploy/elastic-cloud/azure-marketplace-pricing.md
- hidden: deploy/elastic-cloud/create-monthly-pay-as-you-go-subscription-on-gcp-marketplace.md

[florent-leborgne]

Should I unhide all of these as well ?

No, these ones were specifically requested to be hidden (they are also hidden in the asciidoc docs).

For the page added in that PR, I'd let Eric decide

[florent-leborgne]

LGTM from latest comments from Eric