Add installation instructions for MacPorts

[breun]

I am the maintainer of the openjdk*-temurin ports in MacPorts. I recently contributed installation instructions, but I see the website has been replaced with a new version. The instructions can still be found on the old site here: https://old.adoptium.net/installation.html#macports

I looked at the code for the new site, but it seems there is currently a one-to-one mapping from operating system to package manager, which doesn't allow for easily adding a second package manager for an operating system. I'm afraid I don't understand React well enough to refactor the code to allow for instructions for two package managers for a single operating system, but it would be nice if the MacPorts instructions could be restored on the new website.