Address stale docs highlighted in OBO-17

[OisinKyne]

🐞 Bug Report

Description

We reference a component that doesn't exist here. https://github.com/ObolNetwork/charon/blob/v0.18.0/docs/architecture.md#signer

It should likely be removed.

In docs/architecture.md , the architecture overview diagram mentions an internal Signer component that is responsible for
interacting with the external Remote Signer component. However, this internal Signer component does not exist in the codebase. In the
section Stitching the core workflow of the documentation, it also mentions that the Signer.sign function is subscribed to the
consensus client. However, this is not shown in core/interface.go , the file that wires the core workflow together.

Has this worked before in a previous version?

N/A