Fix headers and add !929 docs to extras

[ci skip]

docs/Custom-Emoji.md

+# Custom emoji
+
 To add custom emoji:
 * Add the image file(s) to `priv/static/emoji/custom`
 * In case of conflicts: add the desired shortcode with the path to `config/custom_emoji.txt`, comma-separated and one per line

docs/Message-Rewrite-Facility-configuration.md

+# Message Rewrite Facility configuration
 The Message Rewrite Facility (MRF) is a subsystem that is implemented as a series of hooks that allows the administrator to rewrite or discard messages.
 
 Possible uses include:

mix.exs

@@ -23,11 +23,14 @@ def project do
         logo: "priv/static/static/logo.png",
         extras: [
           "README.md",
-          "docs/config.md",
-          "docs/Pleroma-API.md",
           "docs/Admin-API.md",
           "docs/Clients.md",
-          "docs/Differences-in-MastodonAPI-Responses.md"
+          "docs/config.md",
+          "docs/Custom-Emoji.md",
+          "docs/Differences-in-MastodonAPI-Responses.md",
+          "docs/Message-Rewrite-Facility-configuration.md",
+          "docs/Pleroma-API.md",
+          "docs/static_dir.md"
         ],
         main: "readme",
         output: "priv/static/doc"