Add -Dwarnings to RUSTDOCFLAGS · eclipse-zenoh/zenoh@5460734

The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2, actions-rs/cargo@v1. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

doc tests are not supported for crate type(s) `cdylib` in package `zenoh-plugin-example`

constant `MSG_SIZE_NOFRAG` is never used

`zenoh-transport` (test "unicast_transport") generated 1 warning

unused import: `Priority`

constant `TIMEOUT` is never used

constant `SLEEP` is never used

constant `SLEEP_COUNT` is never used

constant `MSG_COUNT` is never used

constant `MSG_SIZE_NOFRAG` is never used

method `get_count` is never used

struct `TransportMulticastPeer` is never constructed

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