Add note about small upcoming billing change to estimating-spending.md

[holly-kassel]

added link to vnext docs

Why:

Closes: https://github.com/github/gitcoin/issues/18758
This does not close the issue, but deploys the 30-day banner notifying customers of this upcoming change.

What's being changed (if available, include any code snippets, screenshots, or gifs):

A banner will be displayed to customers notifying them of the upcoming changes to how we calculate storage in Billing, linking them to the changelog for more information.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

PR Overview

This PR adds a notification banner to the "estimating-spending.md" document alerting users to an upcoming change in the billing platform. The banner informs users that usage ingestion will switch from minute-based to hourly processing and provides a link to further documentation.

Reviewed Changes

File	Description
content/billing/using-the-new-billing-platform/estimating-spending.md	Added a note section with the upcoming billing change and reference link to vnext docs

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Source	Review	Production	What Changed
billing/using-the-new-billing-platform/estimating-spending.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[nguyenalex836]

Closing per comment here 💛