chore: employ just to run commonly used long cmds[skip ci]

[SteveLauC]

What does this PR do

Closes #2271

I found myself repeatedly opening that issue to copy the command to build the doc:

RUSTDOCFLAGS='--cfg docsrs' cargo +nightly doc --all-features --no-deps --open

Let's wrap these commonly-used commands in a justfile.

This PR adds that doc command, now, to build the doc, one can simply run:

$ just doc

It passes the arguments to the underlying cargo doc (*args=''), so wen can use just doc --open to open the doc.

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API