This adds the `optional` field to the preprocessor configuration to
mirror the same option for the `output` table. Missing preprocessors are
now an error unless the `optional` field is set. This should help with
inadvertently building a book when a missing preprocessor that you
expect to be installed.
restructure guide configuration section
restructure guide configuration section
redirect to new config top level
fix broken links
use a relative path for redirect
Co-authored-by: Eric Huss <eric@huss.org>
remove extra heading
