FINERACT-1956: Add Antora skeleton structure and consolidate contributor guides

[airajena]

Description:

This PR establishes the initial skeleton for publishing Fineract documentation using the Antora static site generator. It consolidates scattered developer guides into a unified "Contributor's Zone" within a standard Antora module structure. This lays the foundation for FINERACT-1959 (migrating content) .

Changes

• New Directory Structure: Created fineract-doc/modules/ with ROOT and contributor modules.
• Antora Configuration: Added fineract-doc/antora.yml component descriptor.
• Module Consolidation: Merged previous development content into the contributor module.
• Content Provisioning: Extracted core developer guides from CONTRIBUTING.md into dedicated AsciiDoc pages:
  • coding-conventions.adoc
  • error-handling.adoc
  • logging-guidelines.adoc
  • checkstyle-and-spotless.adoc
• Cleanup: Refined key-design-principles.adoc to align with current architecture (removed outdated CQRS sections).

Implementation Details

This PR does not replace the existing Asciidoctor Gradle build. The Antora modules exist alongside the current src/docs/en structure, enabling a safe and parallel migration.

Motivation

As discussed in FINERACT-1956, documentation logic was scattered. This "docs as code" structure allows for better versioning, navigation, and maintainability.

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. (Verified standard Antora structure validity)
• 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 - specific to documentation)
• 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.