add default configuration for the pinned statuses and some doc

docs/config.md

@@ -93,6 +93,7 @@ config :pleroma, Pleroma.Mailer,
 * `always_show_subject_input`: When set to false, auto-hide the subject field when it's empty.
 * `extended_nickname_format`: Set to `true` to use extended local nicknames format (allows underscores/dashes). This will break federation with
     older software for theses nicknames.
+* `max_pinned_statuses`: The maximum number of pinned statuses. `0` will disable the feature.
 
 ## :fe
 This section is used to configure Pleroma-FE, unless ``:managed_config`` in ``:instance`` is set to false.

lib/mix/tasks/pleroma/sample_config.eex

@@ -14,7 +14,8 @@ config :pleroma, :instance,
   email: "<%= email %>",
   limit: 5000,
   registrations_open: true,
-  dedupe_media: false
+  dedupe_media: false,
+  max_pinned_statuses: 1
 
 config :pleroma, :media_proxy,
   enabled: false,