docs build failed in google-shopping-merchant-reports

[parthea]

See https://github.com/googleapis/googleapis/blob/492e772e24f4122600c0e106945075a8ff3f6255/google/shopping/merchant/reports/v1beta/reports.proto#L355-L361 where there is a table defined in proto comments.

  // Free listings | Shopping Ads | Status
  // --------------|--------------|------------------------------
  // Approved      | Approved     | ELIGIBLE
  // Approved      | Pending      | ELIGIBLE
  // Approved      | Disapproved  | ELIGIBLE_LIMITED
  // Pending       | Pending      | PENDING
  // Disapproved   | Disapproved  | NOT_ELIGIBLE_OR_DISAPPROVED

This caused the docs build to fail with docstring of google.shopping.merchant_reports_v1beta.types.reports.ProductView.AggregatedReportingContextStatus:5:Undefined substitution referenced: "--------------".

See https://github.com/googleapis/google-cloud-python/actions/runs/6786474451/job/18447083225?pr=11990

In https://google.aip.dev/192#formatting, there is a note that mentions that tables are not allowed in proto comments. Headings and tables must not be used, as these cause problems for several tools, and are unsuitable for client library reference documentation.

We should add a linter rule to check for tables in proto comments via https://github.com/googleapis/api-linter .