Split system requirements by platform

[MelissaAutumn]

The redesign of thunderbird.net plans to display the system requirements per each release note grouped by platform. Currently system requirements are a single blob of text. I'm doing a hacky work-around right now to get the results that's requested, but it would be even better if we just adjusted the source data.

I'll do the work, but if there's any concerns or issues that folks foresee with this change, please let me know! (Here or the upcoming PR.)

Sample system requirement:
https://github.com/thunderbird/thunderbird-notes/blob/master/release/115.0.yml#L17

WIP Redesign:
https://new.thunderbird.net/en-US/thunderbird/115.9.0/releasenotes/