Document multiple CEP stimuli per domain

[samibismar]

Problem

The current CEP Stimulus Subsection presents a single <Stimulus> element and does not explain that the element may be repeated within the same domain.

It also does not describe how multiple stimuli are evaluated and combined, or the relevant default behavior when stimuli or certain parameters are omitted.

Solution

Update the CEP stimulus documentation to describe:

• repeated <Stimulus> elements within an explicit <Domain> or the equation-level default domain;
• independent evaluation of each stimulus using its own amplitude, timing parameters, and optional spatial bounds;
• summation of contributions when multiple stimuli are active at the same node and time;
• zero <Stimulus> elements producing no applied stimulus;
• one <Stimulus> element preserving the previous behavior;
• an omitted or zero-valued <Amplitude> contributing no stimulus;
• an omitted <Cycle_length> using the total simulation duration.

Update the existing Domain and Stimulus structure blocks as needed to indicate that <Stimulus> is repeatable.

Additional context

Follow-up documentation for SimVascular/svMultiPhysics#586.

The relevant documentation source is:

documentation/multi_physics/solver-input-file/domain_parameters/readme.md

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[ktbolt]

@samibismar A very nice Issue description !