docs: write migration guide for v2 to v3

[SethFalco]

We have a migration guide written up for v3 to v4.

However, over 6,600,000 installs a week on npm are for v2 rather than v3/v4. We should maintain a migration guide for v2 to v3 as well to make it easier to ultimately transition to v4.

We can use the release notes for SVGO v3 as the starting point for this.

https://github.com/svg/svgo/releases/tag/v3.0.0

Related

• docs: write migration guide for v1 to v2 #2153