Add 1.2 upgrade guide and spec reference for developers

[elichad]

Adds two new pages to the Resources section:

1. "What's new in RO-Crate 1.2" - An upgrade guide for developers which describes the spec updates chapter by chapter
2. "RO-Crate 1.2 Specification Quick Reference" - A reference page for RO-Crate implementers which concisely lists all of the requirements of the specification.

I was very thorough when making these resources to ensure they are accurate. I did not use AI tools to make them, so every single line has been manually checked and there should not be any hallucinations (unless the process had more of an effect on my psyche than I thought...). Of course, there will probably still be some normal human mistakes :)

You can review these resources rendered on my fork:

https://elichad.github.io/ro-crate/whats-changed-in-1-2
https://elichad.github.io/ro-crate/quick-reference

I am not expecting a full review or cross-checking of these resources since they're long, please just let me know if there are any glaring issues.

The Resources page may not be the best home, open to opinions there, but keen to just get this onto the website now. (I don't want these to be misunderstood as part of the "official" spec, hence avoiding the Specification section)

Fixes #398