Add billing notifications page

[kilfoyle]

This adds docs for the new billing notifications page.

@kyrspl Thanks for opening the parent issue with all of the details! When you know the timing for how the new UI will be rolled out on Serverless and ECH please let me know and I'll update/merge this PR accordingly.

Please see: preview page
Rel: https://github.com/elastic/docs-content-internal/issues/366

Target: Dec.16

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/cloud-organization/billing/manage-billing-notifications.md
• deploy-manage/cloud-organization/billing.md

[shainaraskas]

just a couple small comments :)

[shainaraskas]

just a couple small comments :)

[briannebartlett]

Hey, this page is awesome, thanks! One quick thing I noticed is that for the Configure credit consumption emails section -- this alert is not configurable. It's preset by Elastic and can't be changed. Thanks!

[kilfoyle]

Thanks a lot for catching that @briannebartlett! I've updated the page to make it clear that the credit consumption emails aren't configurable.

[briannebartlett]

@kilfoyle Hey! Quick question, these links are broken now. Can you please take a look?

And also FYI our launch date is December 16th for the first feature release.

[kilfoyle]

Thanks for the heads-up @briannebartlett!

I'll plan to merge this on December 16. Please let me know if there are any changes you'd like. The "preview page" link in the description is working again.

[briannebartlett]

Thank you so much!