Review MarkdownDisplay style markup

[genehack]

The MarkdownDisplay in the SourceInfoHeading is now affected by our globals.css. Looking at https://dev.nextstrain.org/groups/blab-private, it's most obvious looking at the headings:

before

after

Not sure if there's a way to guard against this styling?

Originally posted by @joverlee521 in https://github.com/nextstrain/nextstrain.org/pull/1199#discussion_r2271571838

[emmahodcroft]

Just a note to say this is impacting 'Groups' pages (customized) as it's makes it (by default) not obey any usual .md whitespace options, and instead makes it quite hard to add blank lines! (nbsp; works sometimes)

Would be very nice to have this appear a bit more nicely!