docs: Add installation instructions for MacPorts

[polluks]

Type of change

• Documentation

What does this PR do?

Add installation instructions for MacPorts.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.
• Not all checklist items are checked. Please confirm you have tested locally and have not included unrelated changes.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

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[github-actions]

Hey! Your PR title Add installation instructions for MacPorts doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.