feat(plugins): add OxHugoFlavouredMarkdown (#419)

* feat(plugins): add OxHugoFlavouredMarkdown

ox-hugo is an org exporter backend that exports org files to
hugo-compatible markdown in an opinionated way. This plugin adds some
tweaks to the generated markdown to make it compatible with quartz but
the list of changes applied it is not extensive.

In the future however, we could leapfrog ox-hugo altogether and
create a quartz site directly out of org-roam files. That way we won't
have to do all the ritual dancing that this plugin has to perform.
See https://github.com/k2052/org-to-markdown

* fix: add toml to remarkFrontmatter configuration

* docs: add docs for OxHugoFlavouredMarkdown

* fixup! docs: add docs for OxHugoFlavouredMarkdown