docs: correct SystemVerilog Basics copy

[thomasnormal]

Summary

• Corrects the learner-facing copy in sv/welcome, sv/events, sv/parameters, and sv/enums.
• Aligns event trigger/wait semantics, $bits and unsized literal widths, $clog2, parameter dimensions, and enum typing with IEEE 1800-2023.
• Removes the draft welcome fragment and avoids promising unsupported exhaustive/UVM behavior.

Validation

• npm test -- --run: 84 passed, 4 skipped
• npm run build: passed (existing Svelte accessibility and duplicate-output warnings)
• Native current-tip Mox and Xcelium/refdiff: all four official solutions both_pass
• Focused guard fails on the base copy and passes on this branch