Add decision about the HA model to be used with the s3gw with Longhorn

docs/decisions/0018-s3gw-ha-model.md

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+# s3gw High Availability model
+
+## Context and Problem Statement
+
+We analyzed some High Availability - HA - concepts applied to the s3gw when used with Longhorn.
+The final aim of the research is to identify an HA model we can reasonably rely on.
+
+The full and initial HA research document can be found here:
+[High Availability research](../research/ha/RATIONALE.md).
+You can find there all the rationales, motivations and the details about the tests performed.
+
+## Considered Options
+
+We identified 3 HA models:
+
+- **Active/Active** (multiple s3gw instances concurrently serving the same data)
+- **Active/Warm Standby** (multiple s3gw instances, one serving data, others able to take over if active instance fails)
+- **Active/Standby** (single s3gw instance, with Kubernetes restarting/redeploying as necessary on failure)
+
+## Decision Outcome
+
+The 3 aforementioned models have different performances and different implementation efforts.
+For our use case, the *Active/Standby* model built on top of Longhorn actually makes
+the most sense and brings the "best" HA characteristics relative to implementing a
+more fully active/distributed solution.
+
+List of *desirable* characteristics owned by the *Active/Standby* model
+
+- Simplicity
+- Low implementation effort in respect to the other models
+- Expected to work mainly with Kubernetes primitives
+- Compatible with RWO persistent volume semantics
+- Acceptable restart timings on switch-overs and fail-overs (excluding the non-graceful node failure)
+
+Be aware that the [non-graceful node failure](../research/ha/RATIONALE.md#non-graceful-node-failure)
+problem cannot be entirely solved with the *Active/Standby* model alone.
+Regarding this, we have opened a [dedicated issue](https://github.com/longhorn/longhorn/issues/6803)
+within the Longhorn project.
+
+For a more comprehensive explanation about this choice, please refer to the original
+[High Availability research](https://github.com/aquarist-labs/s3gw/pull/685) pull request.