docs: add frontend development guide for UI customization and navigation

[TalalAhmed31]

As requested in opencti/opencti#12517 , moving this contribution to the official documentation repository.

Proposed changes

• Added a frontend-development-guide file as frontend documentation
• Helps programmers understand the uncommented code and make changes more easily

Related issues

• N/A

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality (documentation build & readability)
• I added/update the relevant documentation (either on github or on notion)

Further comments

• Since the frontend code lacks inline comments, I created this dedicated documentation to help new contributors navigate the codebase efficiently.
• No code logic was modified — this PR is purely documentation-related.

[TalalAhmed31]

Hi @aHenryJard
I’ve updated the guide to remove the hardcoded color values and now reference the constants instead.
Thanks for your helpful feedback — please let me know if any further adjustments are needed.