What's New pages should link to Migration Guides

[allister-beamable]

We have migration guides in the old docs.beamable.com site:

• https://docs.beamable.com/docs/unity-sdk-migration-process for 1.x to 2.x
• https://docs.beamable.com/docs/migration-unity-from-2x-to-30 for 2.x to 3.x

The "What's New" sections in help.beamable.com should link back to those early-version upgrade docs.
For the What's New of the latest version (5.0.1 at time of writing), we should write a brief meta-guide for upgrading from each older version. Practically speaking for 1.x -> 5.x that means "upgrade to 2.x, then to 3.x, then to 5.x (skipping 4)" because of the magnitude (or lack thereof) of upgrade-impacting changes between each pair of adjacent major versions.