Fix various diagnostics typos in settings.mdx and diagnostics.mdx (#115)

* Add missing 'conventions' group to diagnostics group defaults

The conventions group was missing from the default values listed for diagnostics.groupFileStatus and diagnostics.groupSeverity settings.

* Fix 'global-element' typo in diagnostics defaults

Renamed global-elements to global-element in the default values listed for diagnostics.neededFileStatus and diagnostics.severity settings.

* Remove 'cast-type-mismatch' duplicate diagnostic entry

A duplicate cast-type-mismatch diagnostic entry was incorrectly listed in the luadoc group, in addition to the type-check group.

Author: Fringale

src/content/wiki/diagnostics.mdx

@@ -261,22 +261,6 @@ Triggered when referencing an undefined global (assumed to be global). The typic
 
 The luadoc group contains diagnostics for the [annotations](/wiki/annotations) used to document your code.
 
-<Diagnostic level="Warning">
-<Fragment slot="name">
-### cast-type-mismatch
-</Fragment>
-
-Triggered when casting a variable to a type that does not match its initial type.
-
-```Lua
----@type boolean
-local e = nil
-
----@cast e integer
-```
-
-</Diagnostic>
-
 <Diagnostic level="Warning">
 <Fragment slot="name">
 ### circle-doc-class

src/content/wiki/settings.mdx

@@ -266,6 +266,10 @@ Find defined global variables using regex.
    * spell-check
    */
   "codestyle": "Fallback",
+  /*
+   * global-element
+   */
+  "conventions": "Fallback",
   /*
    * duplicate-index
    * duplicate-set-field
@@ -388,6 +392,10 @@ Set the file status required for each diagnostic group. This setting is an `Obje
    * spell-check
    */
   "codestyle": "Fallback",
+  /*
+   * global-element
+   */
+  "conventions": "Fallback",
   /*
    * duplicate-index
    * duplicate-set-field
@@ -534,7 +542,7 @@ Set how files loaded with [`workspace.library`](#workspacelibrary) are diagnosed
   "duplicate-index": "Any",
   "duplicate-set-field": "Opened",
   "empty-block": "Opened",
-  "global-elements": "None",
+  "global-element": "None",
   "global-in-nil-env": "Any",
   "incomplete-signature-doc": "None",
   "inject-field": "Opened",
@@ -622,7 +630,7 @@ Set how files loaded with [`workspace.library`](#workspacelibrary) are diagnosed
   "duplicate-index": "Warning",
   "duplicate-set-field": "Warning",
   "empty-block": "Hint",
-  "global-elements": "Warning",
+  "global-element": "Warning",
   "global-in-nil-env": "Warning",
   "incomplete-signature-doc": "Warning",
   "inject-field": "Warning",