Add to docs install variant via MacPorts

[judaew]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

[docs update] Add install variant via MacPorts to the docs/installation.md

• Does this PR introduce a breaking change, and is
  titled accordingly?

No

• Other information:

I'm a maintainer of the arduino-cli port in MacPorts. I've added the install variant via MacPorts to the docs/installation.md.

Thank you for maintaining this amazing project.

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See how to contribute

[silvanocerza]

Am not sure we want to add download options in the official documentation that we don't maintain ourselves.

I'll leave this up for the time being in case we want to merge in the future.

[cmaglie]

There are many ports of the arduino-cli for various distributions on mac and linux, but we add instructions only for the ones that we directly maintain.