docs: Add OpenMRS wiki reference to Form Builder User Guide in README.md.

[omeriinnocent]

Requirements

• This PR has a title that briefly describes the work done including the ticket number. If there is a ticket, make sure your PR title includes a conventional commit label. See existing PR titles for inspiration.
• My work conforms to the OpenMRS 3.0 Styleguide and design documentation.
• My work includes tests or is validated by existing tests.

Summary

Added wiki page link (https://openmrs.atlassian.net/wiki/x/QgEBCQ) as a bullet point in the Form Builder User Guide section

Why this matters

• Centralizes documentation references for easier contributor onboarding
• Provides direct access to OpenMRS-specific form builder documentation
• Complements existing external guides (Ampath Forms) with internal wiki resources
• Improves discoverability of form builder development guidelines and best practices

Screenshots

Related Issue

https://openmrs.atlassian.net/browse/O3-5463

Other

[omeriinnocent]

@denniskigen and @suubi-joshua kindly review this