docs: write migration guide for v1 to v2

[SethFalco]

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

However, over 5,700,000 installs a week on npm are for v1 rather than v2/v3/v4. We should maintain a migration guide for v1 to v2 as well to make it easier to ultimately transition to v4.

We can use the release notes for SVGO v2 as the starting point for this.

https://github.com/svg/svgo/releases/tag/v2.0.0

Related

• docs: write migration guide for v2 to v3 #2152
• Once complete, consider CLI option to disable plugins is no longer available #1356 (comment)