CI: Add check to test for warnings/errors during building of rust docs

[QuantumDancer]

In #619 I fixed a few error related to the rust docs. A code block was missing, resulting in improper formatting of the docs. However, building the docs threw a few warnings, e.g.:

warning: unresolved link to `gt`
   --> auditor/src/lib.rs:229:147
    |
229 | //! | `start_time` | Start time of the event (`DateTime<Utc>`)                              | `gt`, `gte`, `lt`, `lte`               | start_time[gt]=<timestamp>               |
    |                                                                                                                                                   ^^ no item named `gt` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: unresolved link to `gt`
   --> auditor/src/lib.rs:230:146
    |
230 | //! | `stop_time`  | Stop time of the event (`DateTime<Utc>`)                               | `gt`, `gte`, `lt`, `lte`               | stop_time[gt]=<timestamp>                |
    |                                                                                                                                                  ^^ no item named `gt` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `gt`
   --> auditor/src/lib.rs:231:144
    |
231 | //! | `runtime`    | Runtime of the event (in seconds)                                      | `gt`, `gte`, `lt`, `lte`               | runtime[gt]=<u64>                        |
    |                                                                                                                                                ^^ no item named `gt` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

We should add a CI check that tests if there are any warnings or errors when building the rust docs.
We need to set RUSTDOCFLAGS="-D warnings" (see this stack overflow thread) and then run cargo doc --workspace.