allow indentation in non-vignette md

[maxheld83]

same as in #34.

muggle::build_site2() can only fix this for stuff covered by _site.yml, but not anything else such as README.md or NEWSLETTER.md.

This would have to be solved upstream in pkgdown, by changing how it calls pandoc. Probably same as https://github.com/r-lib/pkgdown/issues/1381