docs : add THEME_DEVELOPMENT.md links to README CONTRIBUTING and THEMES

[tmdeveloper007]

Summary of What Needs to be Done

The THEME_DEVELOPMENT.md file exists in the repo root and contains a comprehensive beginner-friendly theme development guide, but it is not linked from README.md, CONTRIBUTING.md, or THEMES.md. This makes it hard for contributors to discover.

Changes that Need to be Made

Add links to THEME_DEVELOPMENT.md in:

1. README.md - add a link in the Contributing section or a new Theme Development section
2. CONTRIBUTING.md - add a reference in the Theme Design section
3. THEMES.md - add a link or note directing contributors to THEME_DEVELOPMENT.md for theme creation guidelines

Impact that it would Provide

Improves contributor discoverability of theme development resources, leading to more theme contributions and better onboarding for new contributors.

Note: Please assign this issue to the tmdeveloper007 account.

[AI-Explorer786]

/assign