Add default kernel bounds for GP

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Purpose

• Closes Default kernel bounds for GaussianProcess.jl #390

Preview docs here

Content

• add 1e-5 and 1e5 bounds to GP hyperparameters. (in log space)
• make SKLJL upper bound 1e4->1e5
• checked example performance stable
• added tests
• added a docs note for skljl and gpjl

---

• I have read and checked the items on the review checklist.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.20%. Comparing base (d31a2dc) to head (40097aa).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/GaussianProcess.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
+ Coverage   94.18%   94.20%   +0.01%     
==========================================
  Files          10       10              
  Lines        1807     1813       +6     
==========================================
+ Hits         1702     1708       +6     
  Misses        105      105              

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Merging on tests passing, docs checked, and confirmed example performance was stable