FINERACT-2644: Add event-driven modularization design doc (FSIP-12)

[mansi75]

Description

This PR adds a new design document, Event-driven architecture for modular design, to the Fineract documentation (fineract-doc).

The document describes the proposed approach for getting Fineract's module boundaries under control. Today, feature modules import each other's internal classes directly (e.g. loan reads savings entities, savings calls accounting services, accounting depends on organisation repositories), and nothing enforces the use of the existing fineract-core and fineract-command shared modules. The doc proposes making those two modules the only permitted cross-feature dependency — reads cross a boundary as a fineract-core interface returning a DTO, writes cross as a fineract-command command or domain event — and migrating all direct feature-to-feature imports to that model in phased, risk-ordered steps, with an ArchUnit rule activated in CI once the violation count reaches zero.

Changes in this PR:

• Add fineract-doc/src/docs/en/modularization.adoc (the design document).
• Register it in fineract-doc/src/docs/en/index.adoc via an include:: directive so it renders in the documentation build.

This is a documentation-only change. There are no code, REST API, schema, or behavioural changes.

Related JIRA: https://issues.apache.org/jira/browse/FINERACT-2644

Checklist

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Pull request overview

Adds a new AsciiDoc design document describing a proposed event-driven approach to enforcing module boundaries between Fineract feature areas (via fineract-core read contracts and fineract-command write contracts).

Changes:

• Add a new design document: Event-driven architecture for modular design.

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