REX MSDocs validation tool for Python

[JimSuplizio]

Package name on pypi: py2docfx

In the pipeline, download the tool and tool to validate packages during the following parts of the build:

• Package onboarding
• Package metadata updates to docs repo (during release)
• Nightly package docs validation
  Validation process today involves pulling a container image from a private container registry. This step can be removed as the tool replaces the container.

We will pin this package initially to prevent unpredictable breaks. To keep things moving forward on this, add a job to docindex which checks for a later version of the REX tool. If found:

1. Install tool locally
2. Test tool using the shared tests
3. If tests pass open a PR to update the pinned version of the REX tool

[danieljurek]

See also: #34557

[JimSuplizio]

This is fixed with the merge of #38085 in the Python repository.