rustup is not available in termux so, how run documentation locally.
Which is available through rustup doc command?
If you have rust installed, the html files are accessible through $PREFIX/share/doc/rust/html.
You can launch a local web server to read them in your browser, for example:
python -m http.server --directory $PREFIX/share/doc/rust/html
then open 127.0.0.1:8000 in your browser application.
Having rustup in Termux is quite pointless, Termux is not a standard Linux distribution and most of its functionality will not work.

Most helpful comment
If you have
rustinstalled, the html files are accessible through$PREFIX/share/doc/rust/html.You can launch a local web server to read them in your browser, for example:
then open
127.0.0.1:8000in your browser application.Having
rustupin Termux is quite pointless, Termux is not a standard Linux distribution and most of its functionality will not work.