[New article]: [Orleans] Add doc/details for Grain Migration

[ReubenBond]

Proposed topic or title

Live grain migration

Location in table of contents.

Under grains

Reason for the article

We added live grain migration to Orleans, but it is not yet documented

Article abstract

See details here: https://github.com/dotnet/orleans/pull/8452

Detail live grain migration, how it works, where Orleans uses it, how to use it, and how to customize it. Discuss activation repartitioning, or create an article for that, too.

Relevant searches

No response

[ReubenBond]

In the doc, we should talk about migrating grains on-demand via MigrateOnIdle(), directed migration via placement hints, and using IGrainManagementExtension to migrate any grain (https://github.com/dotnet/orleans/blob/3a50ce57d43876a9f7572d529b777c9b1b6a6e05/test/DefaultCluster.Tests/Migration/MigrationTests.cs#L94-L95)

[ReubenBond]

Also IGrainMigrationPartiticpant, how you can implement that on a grain or on another component