chore: Added upgrade-guide for 2.X --> 3.X migration

[michalChrobot]

Purpose of this PR

Adding upgrade-guide in the same way as it was added for N4E. Feel free to make any corrections to it

Jira ticket

N/A

Documentation

Added migration-guide doc

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

N/A

Up-port

N/A

Backports

N/A

[u-pr]

💡 Harness Review

The new guide accurately reflects most source-level breaking changes, but it contains several contradictory upgrade instructions.

_{Reviewed commit 9949c55}

_{🤖 Helpful? 👍/👎}

[michalChrobot]

we can land this after 3.X release

[u-pr]

⚠️ Some settings were ignored in '.pr_agent.toml (repository root)' repo settings

Check the configuration's TOML syntax, permitted settings, and size limit. See the error below.

---

Error message:
Ignored prohibited settings: github_app.handle_pr_actions. Permitted settings were applied.

Configuration content:

# This is the configuration for the u-pr bot
# https://github.cds.internal.unity3d.com/unity/u-pr
# For configuration of this file:
# https://developer.portal.internal.unity.com/catalog/default/component/u-pr/docs/configuration/automatic_runs/

[github_app]
handle_pr_actions = ['opened', 'ready_for_review']  # PR events that auto-run pr_commands
pr_commands       = ["/harness-review"]             # what runs on those events
reviewer_commands = ["/harness-review"]             # what runs when @u-pr is added as reviewer

[jabbacakes]

/review

[u-pr]

💡 Harness Review

The migration guide is generally aligned with the current API and behavior changes, but one documented replacement does not provide a valid migration for the normal legacy call pattern.

_{Reviewed commit 2393dcc}

_{🤖 Helpful? 👍/👎}

[jabbacakes]

Committed directly with changes that bring this upgrade guide in line with the Netcode for Entities one, as well as general style/wording/thoroughness updates.

Ready for review by @NoelStephensUnity and/or @EmandM