add instructions on spelling check using sphinxcontrib-spelling

[zycalice]

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Docs change / refactoring / dependency upgrade

Motivation and Context / Related issue

Add instruction on how to run spell checks using sphinxcontrib.spelling for files build with sphinx.

Related to https://github.com/pytorch/opacus/issues/380

How Has This Been Tested (if it applies)

Checklist

• [x] The documentation is up-to-date with the changes I made.
• [x] I have read the CONTRIBUTING document and completed the CLA (see CONTRIBUTING).
• [ ] All tests passed, and additional code has been covered with new tests.

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@zycalice has updated the pull request. You must reimport the pull request before landing.

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@zycalice has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@zycalice has updated the pull request. You must reimport the pull request before landing.

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@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.