Outdated pieces of vignettes #7755

I pointed an LLM at our vignettes with the intention to find anything obvious we should update, and it found some quick obvious improvements:

Recommendations for Updates

1. Remove Broken Links: Update datatable-intro.Rmd and datatable-reference-semantics.Rmd to either remove references to the "data.table design" vignette or replace them with links to relevant sections in the Wiki or other existing vignettes (like datatable-sd-usage.Rmd or benchmarking).
2. Update Reference Semantics: Update the shallow copy comment in datatable-reference-semantics.Rmd to reflect the current state of internal optimizations and remove the reference to v1.9.8 as a future target.
3. Modernize FAQs: Add the env argument as a recommended solution in FAQ 1.5 and FAQ 2.14, pointing users to the datatable-programming.Rmd vignette for deeper understanding.\

This does not, of course, preclude further improvements missed by this simple one-shot check on documentation quality.