Create v3.6 upgrade guide

[ivanvc]

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The upgrade guide for 3.5 to 3.6 needs to be similar to https://etcd.io/docs/v3.6/upgrades/upgrade_3_5/

We can start with a basic page covering the upgrade steps, which will be similar to 3.5, then we can iterate with additional pull requests to document any breaking changes that users need to be aware of.

From https://github.com/etcd-io/website/issues/926#issuecomment-2676136986.