Add "standalone" section to lowdown.1.
Add "standalone" section to lowdown.1.

Adds a sub-section "Standalone" to lowdown.1 that specifies what happens
when -s is provided.

Also splits out recognised metadata from those within the document, and
those only in -s content.

Future versions will have additional content in "Output modes" that
stipulates any required invocations that a caller may need (this is now
done in e.g. -Tlatex in specifying which includes are necessary).

References https://github.com/kristapsdz/lowdown/issues/78

kristapsdz/lowdown,kristapsdz/lowdown,kristapsdz/lowdown