FINERACT-2458: Add DPG verified badge to main README

[Ambika-Sony]

Description

This PR adds the Digital Public Goods (DPG) verified badge to the project's main README file.
This change was requested by James Dailey to reflect Fineract's status as a verified DPG on the source repository.

Associated JIRA: FINERACT-2458

Checklist

• [x ] Write the commit message as per our guidelines
• [ x] 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.
• [ x] Create/update unit or integration tests for verifying the changes made.
• [x ] Follow our coding conventions.
• [ x] 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
• [x ] 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.

[IOhacker]

@Ambika-Sony please squash and commit the changes, make sure to send only 1 commit per Pull Request.

[meonkeys]

@Ambika-Sony Please have another look at https://github.com/DPGAlliance/dpg-resources/blob/main/docs/dpg-badge-usage.md . The slug should be apache-fineract.

[Ambika-Sony]

@Ambika-Sony please squash and commit the changes, make sure to send only 1 commit per Pull Request.

@IOhacker, thank you for the feedback. I have squashed the changes into a single clean commit as requested. The PR should now be ready for review.

[meonkeys]

the slug looks correct but the link is still broken

[Ambika-Sony]

the slug looks correct but the link is still broken

I have updated the DPG badge to point to the stable organization homepage to avoid the 404 errors found with the deep links. I've also included the apache-fineract slug in the description text as requested. The PR is squashed to a single clean commit. Ready for review