returning the p-th percentile of a slice

Signed-off-by: Nagy Károly Gábriel <k@jpi.io>

Enhance SliceP documentation to clarify behaviour and constraints:
- Document valid range for p parameter (0.0-1.0).
- Clarify in-place sorting behaviour with warning.
- Document return value for invalid inputs.
- Improve example code with better variable naming.

Refactor SliceP tests to comply with TESTING.md requirements:
- Extract test case arrays into factory functions.
- Reduce TestSliceP from 56 lines to 5 lines.
- Add slicePIntTestCases(), slicePFloatTestCases(),
  slicePStringTestCases() returning []TestCase.

Addresses coderabbitai review comments about documentation clarity
and function length compliance.

Signed-off-by: Alejandro Mery <amery@apptly.co>

slices: introduce SliceP()