docs: StorageManager prop corrections

[EthanRBrown]

Description of changes

Corrected the type of the file argument of tthe processFile callback in StorageManager: it was listed as Blob, and should be File.

Also updated CONTRIBUTING.md to add the necessary yarn install step and explained why nvm use is valuable.

Description of how you validated changes

I was not actually able to build or run the docs; the instructions in the Local Development Guides are insufficient. I waded through many issues, but finally had to give up for now...I'll take a look at that another time and if I fix it, I'll submit a PR for that doc update.

Checklist

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• [x] PR description included
• [x] PR title and commit messages follow conventional commit syntax

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.