Provides Skeel condition numbers (condskeel)

- New export `condskeel`
- Documentation with mathematical definition
- Other minor documentation fixes to linalg
Replace tests for linear solver on triangular systems

- Add automatic promotion of integers to floats for condskeel
- Ref. #5605, #5705
Update NEWS.md with condskeel
Merge pull request #5726 from JuliaLang/cjh/fix-5705

Replace magic constant test for linear solve on triangular matrices