docs: Update docs with new envar METAMASK_ENVIRONMENT

[seaona]

Description

• Updates the documentation and js.env.example file with the new variable METAMASK_ENVIRONMENT.
• Adds a small note on the README.md with instructions for updating envars

Related issues

Fixes: #

Manual testing steps

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After

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Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues
• I've included manual testing steps
• I've included screenshots/recordings if applicable
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.
• I’ve properly set the pull request status:
  • In case it's not yet "ready for review", I've set it to "draft".
  • In case it's "ready for review", I've changed it from "draft" to "non-draft".

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[github-actions]

E2E test started on Bitrise: https://app.bitrise.io/app/be69d4368ee7e86d/pipelines/bccf85db-fa0e-4b09-b16c-045cf998071c
You can also kick off another Bitrise E2E smoke test by removing and re-applying the (Run Smoke E2E) label

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[Andepande]

LGTM