fix: disambiguate after grouping by packages and versions

[dandhlee]

If entries need to be disambiguated, it should be done after the entries are grouped into packages/versions to avoid having unique entries to specific version/packages not be disambiguated and left alone without a disambiguation prefix.

New example TOC:

- items:
  - name: Overview
    uid: project-google-cloud-bigquery-storage
  - href: multiprocessing.md
    name: Multiprocessing
  - items:
    - items:
      - name: Overview
        uid: google.cloud.bigquery_storage_v1.client
      - name: BigQueryReadClient
        uid: google.cloud.bigquery_storage_v1.client.BigQueryReadClient
      name: client
      uid: google.cloud.bigquery_storage_v1.client
    - items:
      - name: Overview
        uid: google.cloud.bigquery_storage_v1.reader
      - name: ReadRowsIterable
        uid: google.cloud.bigquery_storage_v1.reader.ReadRowsIterable
      - name: ReadRowsPage
        uid: google.cloud.bigquery_storage_v1.reader.ReadRowsPage
      - name: ReadRowsStream
        uid: google.cloud.bigquery_storage_v1.reader.ReadRowsStream
      name: reader
      uid: google.cloud.bigquery_storage_v1.reader
    - items:
      - name: Overview
        uid: google.cloud.bigquery_storage_v1.services.big_query_read
      - name: BigQueryReadAsyncClient
        uid: google.cloud.bigquery_storage_v1.services.big_query_read.BigQueryReadAsyncClient
      - name: BigQueryReadClient
        uid: google.cloud.bigquery_storage_v1.services.big_query_read.BigQueryReadClient
      name: big_query_read
      uid: google.cloud.bigquery_storage_v1.services.big_query_read

All entries will be stripped off of the prefix unless disambiguation is required, to help shorten entries. Users will still see that they are grouped under specific packages/versions to help tell which packages/versions the entry falls under. From the above example, main difference would be that previously it would have bigquery_storage_v1.client and reader, but now it simply shows client and reader.

Fixes #131

• Tests pass