Documentation: Bad links in docs

[rafilong]

Steps to reproduce

1. Go to Instance Admin API docs
2. Click on ListInstances link in line:

If you want a comprehensive list of all existing instances, make a ListInstances API request with Client.list_instances():

Issue

Docs link to nonexistent repo, which redirects to Java bigtable library repo with nonexistent file.

Where

Faulty link is found in the following docs:

• docs/table-api.rst
• docs/instance-api.rst
• docs/client-intro.rst
• docs/data-api.rst
• docs/row-filters.rst
• tests/unit/test_row_data.py

[rafilong]

On a second glance it looks as though some of these links are valid - previously I had assumed that cloud-bigtable-client had been deprecated, while now it seems as though the repo was renamed to java-bigtable-hbase, and some of these links are still good.