Adding multi cell migration page (#4688)

## What are you changing in this pull request and why?

We are beginning the process of migrating a small set of customers over
to the multicell architecture. This page will be presented in the dbt
Cloud accounts of those slated for migration. It does not need to be
added to the sidebar at this time.

## Checklist

- [ ] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
- [x] For [docs
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#about-versioning),
review how to [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content).
- [x] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

Adding new pages (delete if not applicable):
- [x] Provide a unique filename for the new page

website/docs/docs/cloud/migration.md

@@ -0,0 +1,45 @@
+--- 
+title: "Multi-cell migration checklist"
+id: migration 
+description: "Prepare for account migration to AWS cell-based architecture." 
+pagination_next: null
+pagination_prev: null
+---
+
+dbt Labs is in the process of migrating dbt Cloud to a new _cell-based architecture_. This architecture will be the foundation of dbt Cloud for years to come, and will bring improved scalability, reliability, and security to all customers and users of dbt Cloud.
+
+There is some preparation required to ensure a successful migration.
+
+Migrations are being scheduled on a per-account basis. _If you haven't received any communication (either with a banner or by email) about a migration date, you don't need to take any action at this time._ dbt Labs will share migration date information with you, with appropriate advance notice, before we complete any migration steps in the dbt Cloud backend.
+
+This document outlines the steps that you must take to prevent service disruptions before your environment is migrated over to the cell-based architecture. This will impact areas such as login, IP restrictions, and API access. 
+
+## Pre-migration checklist
+
+Prior to your migration date, your dbt Cloud account admin will need to make some changes to your account.
+
+If your account is scheduled for migration, you will see a banner indicating your migration date when you log in. If you don't see a banner, you don't need to take any action.
+
+1. **IP addresses** — dbt Cloud will be using new IPs to access your warehouse after the migration. Make sure to allow inbound traffic from these IPs in your firewall and include it in any database grants. All six of the IPs below should be added to allowlists.
+    * Old IPs: `52.45.144.63`, `54.81.134.249`, `52.22.161.231`
+    * New IPs: `52.3.77.232`, `3.214.191.130`, `34.233.79.135`
+2. **APIs and integrations** — Each dbt Cloud account will be allocated a static access URL like: `aa000.us1.dbt.com`. You should begin migrating your API access and partner integrations to use the new static subdomain as soon as possible. You can find your access URL on:
+    * Any page where you generate or manage API tokens.
+    * The **Account Settings** > **Account page**.
+
+    :::important Multiple account access
+    Be careful, each account that you have access to will have a different, dedicated [access URL](https://next.docs.getdbt.com/docs/cloud/about-cloud/access-regions-ip-addresses#accessing-your-account).
+    :::
+
+3. **IDE sessions** — Any uncommitted changes in the IDE might be lost during the migration process. dbt Labs _strongly_ encourages you to commit all changes in the IDE before your scheduled migration time.
+4. **User invitations** — Any pending user invitations will be invalidated during the migration. You can resend the invitations once the migration is complete.
+5. **Git integrations** — Integrations with GitHub, GitLab, and Azure DevOps will need to be manually updated. dbt Labs will not be migrating any accounts using these integrations at this time. If you're using one of these integrations and your account is scheduled for migration, please contact support and we will delay your migration.
+6. **SSO integrations** — Integrations with SSO identity providers (IdPs) will need to be manually updated. dbt Labs will not be migrating any accounts using SSO at this time. If you're using one of these integrations and your account is scheduled for migration, please contact support and we will delay your migration.
+
+## Post-migration
+
+After migration, if you completed all the [Pre-migration checklist](#pre-migration-checklist) items, your dbt Cloud resources and jobs will continue to work as they did before. 
+
+You have the option to log in to dbt Cloud at a different URL:
+ * If you were previously logging in at `cloud.getdbt.com`, you should instead plan to login at `us1.dbt.com`. The original URL will still work, but you’ll have to click through to be redirected upon login.
+ * You may also log in directly with your account’s unique [access URL](https://next.docs.getdbt.com/docs/cloud/about-cloud/access-regions-ip-addresses#accessing-your-account).