Installation guide improvements

[beccyabraham]

Increasing Access

Updates to the installation guide would improve clarity for new contributors to the p5 web editor, lowering barriers for contribution.

Feature enhancement details

Some updates to installation.md would improve clarity including:

• More notes on how to use nvm
• Links or screenshots to demonstrate updating developer settings
• Expanding on the instructions to update .env with Github credentials
• Moving the instructions for setting up S3 into the contribution docs and updating them for the new AWS permission settings interface

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