docs: generate the list of options

To generate the list of options, we need to generate and commit a rst
file to make all files available for ReadTheDoc.

An Hydra test ensures this generated file is up-to-date. If it is not
up-to-date, the error message explains the user how to generate it:
the user just needs to run `nix-shell --run generate-rst-options`.

scripts/generate-rst-options.py

@@ -0,0 +1,64 @@
+import json
+import sys
+
+header = """
+Mailserver Options
+==================
+
+mailserver
+~~~~~~~~~~
+
+"""
+
+template = """
+{key}
+{line}
+
+{description}
+
+{type}
+{default}
+"""
+
+f = open(sys.argv[1])
+options = json.load(f)
+
+options = { k: v for k, v in options.items() if k.startswith("mailserver.") }
+
+groups = [ "mailserver.loginAccount",
+           "mailserver.certificate",
+           "mailserver.dkim",
+           "mailserver.fullTextSearch",
+           "mailserver.redis",
+           "mailserver.monitoring",
+           "mailserver.backup",
+           "mailserver.borg" ]
+
+def print_option(name, value):
+    if 'default' in v:
+        if v['default'] == "":
+            default = '- Default: ``""``'
+        else:
+            default = '- Default: ``{}``'.format(v['default'])
+    else:
+        default = ""
+    print(template.format(
+        key=k,
+        line="-"*len(k),
+        description=v['description'],
+        type="- Type: ``{}``".format(v['type']),
+        default=default))
+
+print(header)
+for k, v in options.items():
+    if any([k.startswith(c) for c in groups]):
+        continue
+    print_option(k, v)
+
+for c in groups:
+    print(c)
+    print("~"*len(c))
+    print()
+    for k, v in options.items():
+        if k.startswith(c):
+            print_option(k, v)