fix: rust doc.rs docstring build

[LVivona]

Summary

when building to doc.rs it seems to be unable to find the docs/katex.html, so I moved it within the bintensors directory.

...
[INFO] [stderr] warning: target filter specified, but no targets matched; this is a no-op
[INFO] [stderr]  Documenting bintensors v0.0.1-alpha.2 (/opt/rustwide/workdir)
[INFO] [stderr] error: error reading `../docs/katex.html`: No such file or directory (os error 2)
...