Remove `.env.example`, update `CONTRIBUTING.md`

[austin-denoble]

Problem

Within the SDK itself we're not reading anything from an .env file, but the repo still has an example file that's referenced in the CONTRIBUTING.md file.

Solution

Remove .env.example and update the section about setting up your environment in the CONTRIBUTING.md file.

Type of Change

• [ ] 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 not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [X] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

N/A