have layout default to `md.jlmd` or `layout.jlhtml`

Unless someone wants some fancy layout, it's reasonable to assume that 99% of markdown files will use `md.jlmd` and 99% of pluto notebooks will use `layout.jlhtml`.

Those could be the default values, so that one would need to specify the layout in the frontmatter only if they want something fancier

Locate the code that reads frontmatter and selects layouts for Markdown files and Pluto notebooks. Check how missing layout values are handled, then verify that md.jlmd and layout.jlhtml are used as defaults while explicit frontmatter values still override them.

Written by the indexing model from the issue text.