Markdown code blocks part 2; migrate Nim Manual (#20080)

* Change headings underscored by `~~~` to `###`

* Markdown code blocks part 2; migrate Nim Manual

tests/stdlib/trst.nim

@@ -471,21 +471,34 @@ suite "RST parsing":
                   rnLeaf  '.'
           """)
 
+  let expectCodeBlock = dedent"""
+      rnCodeBlock
+        [nil]
+        [nil]
+        rnLiteralBlock
+          rnLeaf  '
+      let a = 1
+      ```'
+      """
+
   test "Markdown code blocks with more > 3 backticks":
     check(dedent"""
         ````
         let a = 1
         ```
-        ````""".toAst ==
-      dedent"""
-        rnCodeBlock
-          [nil]
-          [nil]
-          rnLiteralBlock
-            rnLeaf  '
+        ````""".toAst == expectCodeBlock)
+
+  test "Markdown code blocks with ~~~":
+    check(dedent"""
+        ~~~
         let a = 1
-        ```'
-      """)
+        ```
+        ~~~""".toAst == expectCodeBlock)
+    check(dedent"""
+        ~~~~~
+        let a = 1
+        ```
+        ~~~~~""".toAst == expectCodeBlock)
 
   test "Markdown code blocks with Nim-specific arguments":
     check(dedent"""

tests/stdlib/trstgen.nim

@@ -552,7 +552,7 @@ context1
 
 context2
 """
-    let output1 = input1.toHtml
+    let output1 = input1.toHtml(preferRst)
     doAssert "<hr" in output1
 
     let input2 = """
@@ -1616,3 +1616,8 @@ suite "local file inclusion":
     discard ".. code-block:: nim\n    :file: ./readme.md".toHtml(error=error)
     check(error[] == "input(2, 20) Error: disabled directive: 'file'")
 
+  test "code-block file directive is disabled - Markdown":
+    var error = new string
+    discard "```nim file = ./readme.md\n```".toHtml(error=error)
+    check(error[] == "input(1, 23) Error: disabled directive: 'file'")
+