Plugins: added Resource CircularChart example plugin

[Faakhir30]

Closes #586 [READY FOR REVIEW]

Implements a Resource CircularChart example plugin showing pod failure stats

This PR adds a new example plugin that demonstrates how to use Headlamp's CircularChart components to visualize cluster resource data. The implementation process involved:

1. Created a new resource-charts plugin showing failed vs total pods

2. Initially used CircularChart component, then simplified to use TileChart directly:

   • Removed unnecessary metrics-related functionality
   • Added theme-based error coloring for better visualization
   • Improved chart labeling and percentage calculations

3. Added documentation in README.md explaining:

   • Plugin purpose and features
   • Setup instructions
   • Implementation details and key concepts

The example plugin is:

• Using K8s.ResourceClasses for data fetching
• Implementing chart processors for Overview page as an example page.
• Using redux slice for plugin charts, then processing on frontend code.

Pod Failed Chart on Overview Page:

Plugin Details:

Screencast of failing live Pod:

Screencast.from.2025-02-03.02-45-13.webm

Tests Passing (YES :)

[illume]

Nice one.

I noticed there's an issue when building docs on CI: https://github.com/headlamp-k8s/headlamp/actions/runs/13102964037/job/36701030284?pr=2832#step:4:28

Probably you can make docs to reproduce it locally.

[illume]

btw, we don't use the feat() style commit messages.

[Faakhir30]

Thanks @illume ,
Reproduced and fixed the failing check.

Rebased with upstream main and cleared commit history to single commit with specified style of commits in headlamp.