chore(ui): add ui guide for developers

[chirag-madlani]

This pull request introduces comprehensive developer documentation for the OpenMetadata UI codebase. It adds a dedicated developer handbook detailing folder structure, naming conventions, component and type organization, API and state management practices, styling guidelines (with LESS and MUI), icon usage, and testing standards. Additionally, the main README.md is updated to reference this handbook and provide onboarding instructions for new contributors.

Documentation improvements:

• Added a new DEVELOPER_HANDBOOK.md file outlining folder structure, naming conventions, component and type patterns, API/state management, styling with LESS and MUI, icon usage with @untitledui/icons, and testing standards for the UI codebase.
• Updated README.md to include a "Developer instructions" section, linking to the new handbook and summarizing key technologies and onboarding steps for new developers.

Describe your changes:

Fixes

I worked on ... because ...

Type of change:

• Bug fix
• Improvement
• New feature
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

Checklist:

• I have read the CONTRIBUTING document.
• My PR title is Fixes <issue-number>: <short explanation>
• I have commented on my code, particularly in hard-to-understand areas.
• For JSON Schema changes: I updated the migration scripts or explained why it is not needed.

[github-actions]

Jest test Coverage

UI tests summary

Lines	Statements	Branches	Functions
	63.43% (49461/77981)	40.5% (23646/58383)	44.29% (7494/16921)

[sonarqubecloud]

Quality Gate passed for 'open-metadata-ui'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud