disable cargo doc for subxt-cli

[niklasad1]

Currently we are sometimes running into rust-lang/cargo#6313 because subxt crate and the binary name for subxt-cli causes a file-name collision when running cargo doc.

This PR disables doc for subxt-cli crate which is not that useful anyway. The CLI --help should be sufficient

Fixes the following warning from the cargo doc:

warning: output filename collision.
The bin target `subxt` in package `subxt-cli v0.34.0 (/home/niklasad1/Github/subxt/cli)` has the same output filename as the lib target `subxt` in package `subxt v0.34.0 (/home/niklasad1/Github/subxt/subxt)`.
Colliding filename is: /home/niklasad1/Github/subxt/target/doc/subxt/index.html
The targets should have unique names.
This is a known bug where multiple crates with the same name use
the same path; see <https://github.com/rust-lang/cargo/issues/6313>.

[tadeohepperle]

👍