[ADP-3092] Add documentation "When to use cardano-wallet" (#4041)

This pull request adds a documentation section "When to use
`cardano-wallet`" that describes in more detail when you would want to
choose `cardano-wallet`.

docs/site/src/SUMMARY.md

@@ -1,5 +1,6 @@
 - [Introduction](index.md)
 - [User Manual](user.md)
+    - [When to use](user/when.md)
     - [How to](user/common-use-cases.md)
         - [Start a server](user/common-use-cases/start-wallet-server.md)
         - [Create a wallet](user/common-use-cases/create-a-wallet.md)

docs/site/src/user/when.md

@@ -0,0 +1,60 @@
+# When to use `cardano-wallet`
+
+Cardano-wallet was originally designed to provide the wallet logic for the graphical frontend [Daedalus][]. Cardano-wallet does not offer a graphical interface, but it provides an HTTP API and a command line interface (CLI).
+
+Cardano-wallet is a **full-node wallet**, which means that it depends on a `cardano-node` to provide blockchain data. In turn, this means that `cardano-wallet` enjoys the security properties of the Ouroboros consensus protocol. Other wallets such as [Eternl][], [Yoroi][] or [Lace][] are light wallets, which means that they have to trust a third party to provide blockchain data, but they use less computing resources in return.
+
+Cardano-wallet supports
+
+* All use cases
+
+    * Managing a balance of ADA and tokens
+    * Submitting transactions to the Cardano network via a local `cardano-node``
+
+* Personal use
+
+    * Staking
+    * Compatibility with legacy wallets
+    * Basic privacy with payment addresses creation
+
+* Business use
+
+    * Minting and burning tokens
+    * Multi-party signatures
+
+We also accomodate
+
+* Cryptocurrency Exchanges
+
+  [Daedalus]: https://daedaluswallet.io/
+  [Eternl]: https://eternl.io/
+  [Lace]: https://www.lace.io/
+  [Yoroi]: https://yoroi-wallet.com/
+
+Please [reach out to us][forum] if you feel that we could do a better job of covering your use case. Preferably, tell us more about what you are trying to achieve and the problems you want to solve — then we can help you find out whether `cardano-wallet` is the right tool for you, and we can better accomodate your use case in our development roadmap.
+
+  [forum]: https://github.com/cardano-foundation/cardano-wallet/discussions
+
+## Scalability
+
+### Computing resources
+
+A single `cardano-wallet` process supports muliple wallets, each with separate (sets of) signing keys. These wallets run fairly independently, so that the computing resources used by `cardano-wallet` will be the sum of the resources used by each wallet.
+
+Each wallet uses computing resources that grow with the number of addresses and UTxO in the wallet. For precise numbers, see our [Hardware Recommendations](user/hardware-recommendations.md).
+
+If computing resources on a single machine are not sufficient, you can run multiple `cardano-wallet` process with different wallets on separate machines.
+
+### Transaction throughput
+
+If you want to submit frequent transactions to the Carano blockchain, the wallet is typically not a bottleneck — rather, the transactions per seconds that can be accepted by the Cardano blockchain is the limiting factor. That said, transactions per second are not directly relevant to your end-users, see [Performance engineering: Lies, damned lies and (TPS) benchmarks][neil] for a thorough discussion of performance on Cardano.
+
+In general, note that the more your transactions depend on each other, the less they can be processed in parellel, reducing throughput and increasing latency.
+
+On Cardano mainnet, on average 1 block per 20 slots is made, and one slot lasts 2 seconds (parameters as of July 2023). Each [block may contain a maximum number of transactions][block-size]. Using these quantities, you can estimate an upper bound on the number of your transactions per second that the blockchain can accomodate.
+
+If you need more frequent transactions that the estimate above, you need to consider a scaling solution such as [Hydra][] or a sidechain.
+
+  [block-size]: https://cardano.stackexchange.com/questions/216/is-there-a-maximum-number-of-transactions-a-block-can-hold 
+  [Hydra]: https://hydra.family/
+  [neil]: https://www.youtube.com/watch?v=gpSnyCn2s9U