Consider making Discord link easier to find

Links to the official Discord server for discussion/help/contribution are currently only in the footer and help page (see a21702becc1a3fd3f5553a10242f627124d956e9). Originally I just wanted to replace existing links for Reactiflux (Discord) and Spectrum, but I think a lot of users are missing them as there are still issues in this GitHub organization that are more help questions, which are easier to discuss on Discord.

I'd appreciate ideas for how to make it easier to find. I think the main issue is that each package has a GitHub link but not a Discord link. Maybe we should add it to the navbar or each library page that has a channel (which should be most of them). I also need some help with the layout issue in https://github.com/testing-library/testing-library-docs/pull/507 so we can remove the Spectrum link from the help page, making the Discord link a little higher priority.

Start by reviewing the current Discord and Spectrum links in the footer and help page, the referenced commit, and the layout discussion in testing-library-docs#507. The issue presents several possible placements rather than a decided change. Done would require an agreed location or locations, updated documentation navigation or library pages, and verification that the links are easier to find.