Create setting for default type of draft tiddlers (#8783)

* Proof of concept to change type of tiddlers created by editing missing.

* Adding a setting for default type so that it can be overridden for other wikis.

* Incorporating PR feedback.

* Adding documentation tiddler for new setting.

* Added documentation to the markdown plugin documentation.

* Added documentation to the markdown plugin documentation.

* Added notice to markdown edition.

plugins/tiddlywiki/markdown/readme.tid

@@ -98,4 +98,22 @@ When [[typographer|$:/config/markdown/typographer]] is enabled, markdown-it will
 ,,        → ,
 --        → –
 ---       → —
-```
+```
+
+! Making Markdown the Default in TiddlyWiki
+
+!! Modifying Existing Actions
+
+Adding `type="text/markdown"` to the actions of the following buttons will
+override the default behavior.
+
+* New Journal - $:/core/ui/Actions/new-journal
+* New Tiddler Here - $:/core/ui/Buttons/new-here
+* New Tiddler - $:/core/ui/Actions/new-tiddler
+
+!! Using Markdown for Missing Links
+
+By default, missing tiddlers are created with a blank content type, which
+implies WikiText. Setting [[$:/config/DefaultMissingType]] to `text/markdown`
+will make it so that tiddlers created from a missing tiddler link will be in
+Markdown rather than in WikiText.