docs: add introduction for multi-project migration

What
---
Add introduction for migration.

Why
---
It was suggested in
!164 (comment 1435952068)
to add a summary of the document.

Reference: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/23961


Signed-off-by: Steve Xuereb <sxuereb@gitlab.com>

library/gitaly-multi-project/README.md

@@ -4,6 +4,22 @@ Epic: <https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/935>
 
 [[_TOC_]]
 
+## Introduction
+
+As part of the [Disaster Recovery Working Group](https://about.gitlab.com/company/team/structure/working-groups/disaster-recovery/)
+we will be splitting the Gitaly VMs across multiple GCP projects. Each project will have 9 Gitaly VMs spread evenly between 3 zones.
+
+This is to prevent network throttling when we are taking snapshots and
+recovering from snapshots, and multiple projects will give us a high number of
+concurrency on snapshots, which will improve our RTO target.
+
+We will be migrating project repositories from the current Gitaly VMs to the
+new ones with no downtime. We will have a service running the migration in
+the background to drain storages 1 by 1, and we'll have the possibility to
+increase the concurrency of how many repositories we are migrating. When we are
+moving a repository from the current storage to the new one it will be
+temporarily set to read-only.
+
 ## Architecture
 
 <details>