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.

[MBoegers]

We use Asciidoctor to generate the documentation part, which generates static HTML.
As we ported the documentation from the old website we, or better I, found no easy way to replace names (like openjdk17-temurin) based on a selection.

You could:

• add your instruction to the MacOS installation page. Similar to the yum and apt sections at the linux installation page
• add your instruction as new macport.adoc page and link the page in the installation idex.html

If you have any problems, feel free to reach out here or via slack :-)