Fix executable copying on build

[iinuwa]

• correct documentation on executable location
• use hard links to executable to save space.

[iinuwa]

I used a hard link to save space. I wanted to see if we could use the dep files generated by Cargo in the Meson targets to also avoid rebuilds, but I couldn't figure out a deterministic path to the dep file. (A fingerprint suffix is always added to the dep file, and I don't know when or why that changes.)