docs: Reinstating UNET page (#3667)

## Purpose of this PR
Reinstating the UNET migration page 

### Jira ticket
[MTT-13095](https://jira.unity3d.com/browse/MTT-13095)

### Changelog

- Added: Documentation about migrating from UNET to Netcode for
GameObjects.

## Documentation

- Added new public manual documentation.

## Testing & QA (How your changes can be verified during release
Playtest)

Built locally to check everything works.

## Backports

#3668

Author: jabbacakes

com.unity.netcode.gameobjects/Documentation~/TableOfContents.md

@@ -90,3 +90,4 @@
     * [Optimizing Boss Room](samples/bossroom/optimizing-bossroom.md)
     * [NetworkRigidbody](samples/bossroom/networkrigidbody.md)
     * [Spawn NetworkObjects](samples/bossroom/spawn-networkobjects.md)
+* [Migrate from UNet to Netcode for GameObjects](migratingfromUNet.md)