files getting excluded when regenerating index.rst

[vchudnov-g]

There are three kinds of erroneous exclusions from index.rst we noticed when reviewing https://github.com/googleapis/google-cloud-python/pull/11635. The changes I had to manually make to fix these exclusions are shown here. We should fix our processes so this kind of manual fix is not needed. The omissions were:

• sibling packages were excluded. In this case, the lines referring to executions below should have been present but weren't:

   workflows_v1/services
   executions_v1/services
   workflows_v1/types
   executions_v1/types
  

• the "Changelog" section and its reference to the CHANGELOG file were excluded
• the .. include:: directives were excluded

At least the last two of these exclusions were caught by docs presubmit failures, but they should not be happening at all.

(This issue may need to be moved to the owlbot repo, or link to new issues there.)

[vchudnov-g]

Labeling this P1 because of the toil involved.

[vchudnov-g]

Lowering to P2 because we have a manual workaround when this arises, and it's not blocking releases. Our upcoming docs work will include automating this.