Fix doc warnings and link failures (#1948)

Also add configuration to CI to fail doc generation if any links are
broken. Unfortunately we can't blanket deny all warnings in rustdoc
since some are unconditional warnings, but for now this is hopefully
good enough.

Closes #1947

crates/runtime/src/externref.rs

@@ -21,7 +21,7 @@
 //! The `VMExternData` struct is *preceded* by the dynamically-sized value boxed
 //! up and referenced by one or more `VMExternRef`s:
 //!
-//! ```ignore
+//! ```text
 //!      ,-------------------------------------------------------.
 //!      |                                                       |
 //!      V                                                       |

crates/wiggle/src/lib.rs

@@ -61,7 +61,7 @@ pub use region::Region;
 /// See the safety guarantees of [`BorrowChecker`], which asserts that exactly
 /// one `BorrowChecker` may be constructed for each WebAssembly memory.
 ///
-/// The [`GuestMemory::as_slice`] or [`GuestPtr::as_str`] will return smart
+/// The [`GuestPtr::as_slice`] or [`GuestPtr::as_str`] will return smart
 /// pointers [`GuestSlice`] and [`GuestStr`]. These types, which implement
 /// [`std::ops::Deref`] and [`std::ops::DerefMut`], provide mutable references
 /// into the memory region given by a `GuestMemory`.

crates/wiggle/wasmtime/macro/src/lib.rs

@@ -21,7 +21,7 @@ use config::{MissingMemoryConf, ModuleConf, TargetConf};
 ///   where the macro is invoked. `witx_literal` takes a string of the witx document, e.g.
 ///   `"(typename $foo u8)"`.
 /// * `ctx`: The context struct used for the Wiggle implementation. This must be the same
-///   type as the [`wasmtime_wiggle::from_witx`] macro at `target` was invoked with. However, it
+///   type as the `wasmtime_wiggle::from_witx` macro at `target` was invoked with. However, it
 ///   must be imported to the current scope so that it is a bare identifier e.g. `CtxType`, not
 ///   `path::to::CtxType`.
 /// * `modules`: Describes how any modules in the witx document will be implemented as Wasmtime