Don't always add margin-top to headers

[cpsievert]

Closes #375

[gadenbuie]

I updated the SCSS rules to apply to .section.levelX and section.levelX so that they apply in both html4 and html5 settings with Bootstrap 5. (To see this, set pandoc_args = c("--to", "html5").)

Also, it's worth noting that when section_divs = FALSE (with version = 5), our styles won't apply and the resulting headings won't have top margin since Bootstrap's default styles will apply. In general, I think this is the right way to go: most prose settings will benefit from the additional top margin, without interfering with the use of headings in other places.

[cpsievert]

NOTE TO SELF:

• Before merging, review CSS that we've written to work around this issue (e.g., cards and accordions) and remove unnecessary workarounds
• Document the breaking change, and for users of shiny::titlePanel(), recommend using (tagAppendAttributes(x, class = "mt-3", .selector = "h2")