📝 Overhaul README

[thisiskeithb]

As the title states, completely overhaul the repository README.

The main focus (and entire reason I started down this path) was to add tested and working Ruby & Jekyll install procedures for local development/preview since the current instructions were woefully out of date and did not work.

This involved setting up new VMs for Windows 10, macOS 12.7.6 (limited by my hardware), & Ubuntu 24.04 LTS and validating each step to work out any bugs. After working out the steps & commands, VMs were reset to a fresh state & the updated Ruby/Jekyll install process was verified to work without issue.

H/t to @ellensp for being a sounding board while I bashed my head against the wall working through these issues.

[thisiskeithb]

This PR now relies on #579 since I've confirmed that everything builds in clean installs of Ubuntu 24.04 LTS, Windows 10, and macOS Monterey 12.7.6 (limited by my hardware) using Jekyll 3.10.0 & Ruby 3.3.4. Instructions have been updated accordingly.

I've also added Ubuntu instructions based on @ellensp's tips.

[thinkyhead]

Homebrew is a popular macOS package manager, but I prefer to use MacPorts, and I wrote my earlier instructions with consideration for both the MacPorts package manager and Homebrew. It would be good to retain the instructions for both.

[thisiskeithb]

It would be good to retain the instructions for both.

Noted. I'll verify the MacPorts instructions still work & adjust as necessary.

[thisiskeithb]

I'll verify the MacPorts instructions still work & adjust as necessary.

We didn't exactly have complete MacPorts installation instructions before, but I tested & confirmed that the updated instructions work.

I vaguely remember something about paths being different if you're on an Arm-based Mac, so I need to double-check that and potentially split things up further.