Revert "Enable custom header/footer templates for DocC."

This reverts commit e7029a6 to avoid a
merge conflict with 'main', where the makefile was deleted.

Author: amartini51

Makefile

@@ -10,11 +10,11 @@
 .IGNORE: preview
 .PHONY: preview
 preview:
-	swift package --disable-sandbox preview-documentation --experimental-enable-custom-templates
+	swift package --disable-sandbox preview-documentation
 
 .PHONY: archive
 archive: .build/plugins/Swift-DocC/outputs/TSPL.doccarchive
 
 .build/plugins/Swift-DocC/outputs/TSPL.doccarchive: Sources/TSPL/TSPL.docc/*/*.md
 .build/plugins/Swift-DocC/outputs/TSPL.doccarchive: Sources/TSPL/TSPL.docc/Assets/*.png
-	swift package generate-documentation --experimental-enable-custom-templates
+	swift package generate-documentation