DOC: start new section about output format

doc/Doxyfile

@@ -6,7 +6,7 @@ CREATE_SUBDIRS         = NO
 OUTPUT_LANGUAGE        = English
 GENERATE_LATEX         = NO
 FULL_PATH_NAMES        = NO
-INPUT                  = ../src api_intro.md papers.md start_dev.md formats.md installation.md interaction.md spc_format.md verification.md
+INPUT                  = ../src api_intro.md papers.md start_dev.md formats.md installation.md interaction.md output.md spc_format.md verification.md
 OPTIMIZE_OUTPUT_FOR_C  = YES
 SOURCE_BROWSER         = NO
 VERBATIM_HEADERS       = NO

doc/output.md

@@ -0,0 +1,9 @@
+Output format
+=============
+
+The output of gr1c can be presented in JSON if the command-line switch --json is
+provided. This page describes versions of the format.  Like the [file
+formats](./formats.md), they are numbered separately from versions of gr1c
+itself.
+
+<h2 id="gr1coutv0">version 0</h2>