sidekick: generate cross-reference links in documentation comments

The documentation comments include things like:

```swift
/// Creates a new
/// [ExternalAccountKey][google.cloud.security.publicca.v1.ExternalAccountKey] in
/// a given project.
```

The generator should enrich the comments with reference-style links:

```swift
/// Creates a new
/// [ExternalAccountKey][google.cloud.security.publicca.v1.ExternalAccountKey] in
/// a given project.
///
/// [google.cloud.security.publicca.v1.ExternalAccountKey]: `ExternalAccountKey`
```

*OR* (if this is easier), convert the cross-reference links in place:

```swift
/// Creates a new `ExternalAccountKey` in a given project.
```

The first approach is used by Rust, the second approach is used by Dart.

The first approach is known to work with external types. Maybe the second one does too?