Improve splineProperties() documentation

[Homaid]

Resolves #8260

Changes:

1. Added two example sketches demonstrating tightness and ends properties
   

2. Expanded description to explain common properties
   

3. Clarified relationship with spline(), splineProperty(), and splineVertex()
   

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

[Homaid]

hey @perminder-17 would love your review on this

[perminder-17]

I took a quick look at the docs and they look great. I’ll run the site locally to review them more carefully, and if everything works as expected, I’ll go ahead and merge.

Thanks for your work on this, @Homaid.