Add ⚡️ content to `Designing Bitcoin Products` > `User research resources`

[GBKS]

Add Lightning-related content to the User research resources page, as appropriate. This page likely needs only minor tweaks. See notes from Jam Session #14.

Secondary, do a general review of this page (check overlap with other pages, add relevant cross-links...) and check open issues for changes on this page and implement them.

[Bosch-0]

Going to add https://static1.squarespace.com/static/60377b34e7791c1277aaae97/t/615cb9aeaab8c63e1a732771/1633466815710/The+State+of+Lightning.pdf to this page. We can just add content to this section as we find it, could close this issue.