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

Please make sure these boxes are checked before submitting your pull request - thanks!

• Write the commit message as per our guidelines
• Acknowledge that we will not review PRs that are not passing the build ("green") - it is your responsibility to get a proposed PR to pass the build, not primarily the project's maintainers.
• Create/update unit or integration tests for verifying the changes made. — N/A: documentation-only change, no code paths to test. Verified locally with ./gradlew :fineract-doc:doc (docs build green).
• Follow our coding conventions.
• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes — N/A: no API changes.
• This PR must not be a "code dump". Large changes can be made in a branch, with assistance. Ask for help on the developer mailing list.

Your assigned reviewer(s) will follow our guidelines for code reviews.

[Copilot]

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.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[adamsaghy]

We dont have any ArchUnit lib or rules...

[adamsaghy]

Neither Spring Modulith is not used.

[mansi75]

I have added the Spring Modulith for cross-feature boundary test so I included that in the document also since we are going forward with the documentation along with the implementation. I will update it accordingly

[adamsaghy]

@mansi75 @vidakovic This modularization.adoc is describing situations and libs which not exists in Fineract. Should we first start the work before having this doc?