Add MacPorts Documentation

[harens]

Make sure that you've checked the boxes below before you submit PR:

Closes #402.

For MacPorts, I included how to get shell completion and noted the project page.

I also changed Mac --> macOS and added a repology badge. Finally, I changed the Linux instructions syntax highlighting from text to sh.

Always

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• Written well with PR title, we generate the release notes base on that

For the bug fixes or features only

• Quality Gate Passed. Change this URL to your PR.
• The coverage is xxx on the new lines
• I've tested it by manual in the following platform
  • MacOS
  • Linux
  • Windows

[LinuxSuRen]

/kind doc

[LinuxSuRen]

/lgtm

[LinuxSuRen]

/approve

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[harens]

Thanks @LinuxSuRen. Great automation with the bot btw.

[LinuxSuRen]

Automation bot is helpful and useful.