sidekick: goldmark is still missing some cases of html tags

The markdown comments in googleapis include HTML-looking tags, such as ``. These must be escaped or Rust complains (currently we disable some warnings in Rust and potentially miss other problems).

Sidekick uses `goldmark` to escape the HTML-looking tags to `\`. It is missing some (an example is below. We should fix this problem.

----

Generating `google/cloud/aiplatform/v1` and calling `cargo doc` is resulting in the following issue:
```
warning: unclosed HTML tag `annotation-type`
--> src/generated/cloud/aiplatform/v1/src/model.rs:71565:60
|
71565 | /// "bigquery_destination.dataset_\__\.validation"
| ^^^^^^^^^^^^^^^^^
|
help: try marking as source code
|
71565 | /// "bigquery_destination.dataset_\`_`_\.validation"
| + +

warning: unclosed HTML tag `annotation-type`
--> src/generated/cloud/aiplatform/v1/src/model.rs:71568:60
|
71568 | /// "bigquery_destination.dataset_\__\.test"
| ^^^^^^^^^^^^^^^^^
|
help: try marking as source code
|
71568 | /// "bigquery_destination.dataset_\`_`_\.test"
| + +

```

Interestingly, `"gcs_destination/dataset---/test-*.${AIP_DATA_FORMAT}"` is correctly getting escaped.

Start by reproducing generation for google/cloud/aiplatform/v1 and running cargo doc, then trace Sidekick's goldmark HTML-escaping path. Compare the failing annotation-type and time examples with the correctly escaped gcs_destination example; done means the generated Rust documentation produces no corresponding unclosed HTML tag warnings.

Written by the indexing model from the issue text.