How-To Guide: on how to sign contract invocations using C-accounts (#2206)

### What

How-to guide on how to sign contract invocations using G-accounts or C-accounts.

Although the guide covers both signing types (transaction signing, and auth-entry signing), it focuses more heavily on the auth-entry signing, which is new(er), more complicated and lacks documentation.

### Why

Having the opportunity to interact with multiple projects that enforce that flow (Launchtube, OpenZeppelin Relayer, WalletBackend, MeridianPay), I know very well how that's poorly documented and has many pitfalls.

This guide provides working code examples using `AssembledTransaction`, explains simulation and auth requirements, and includes visual diagrams to clarify the authorization flow.

### AI

The guide and snippets have been tested by multiple AI agents for functionality and clarity ✅

### Preview

Should be deployed to:
https://developers-pr2206.previews.kube001.services.stellar-ops.com/docs/build/guides/transactions/signing-soroban-invocations

Author: marcelosalloum

docs/build/guides/transactions/fee-bump-transactions.mdx

@@ -11,6 +11,8 @@ A fee-bump transaction is made of two parts:
 1. An inner transaction envelope with its signature(s)
 2. An outer transaction envelope with the fee-bump transaction and fee account signature
 
+![Fee-bump transaction structure](/assets/guides/fee-bump-transaction.png)
+
 ## Common use cases
 
 You may want to consider using fee bumps when: