Add ffmpeg install for doctests

[amyeroberts]

What does this PR do?

tests_pr_documentation_tests fails if run on some docs e.g. docs/source/en/task_summary.mdx as ffmpeg is not installed.

Install ffmpeg in pr_documentation_tests CircleCI job.

Despite trying to add changes to the code and docstrings - I was unable to trigger the tests in the CI suite for the doc tests :(

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[HuggingFaceDocBuilderDev]

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