Markdown indented code blocks (#20473)

* Implement Markdown indented code blocks

Additional indentation of 4 spaces makes a block an "indented code block"
(monospaced text without syntax highlighting).
Also `::` RST syntax for code blocks is disabled.

So instead of
```rst
see::

  Some code
```

the code block should be written as
```markdown
see:

    Some code
```

* Migrate RST literal blocks :: to Markdown's ones

doc/filters.md

@@ -10,15 +10,15 @@ A `Source Code Filter (SCF)`  transforms the input character stream to an in-mem
 output stream before parsing. A filter can be used to provide templating
 systems or preprocessors.
 
-To use a filter for a source file the `#?` notation is used::
+To use a filter for a source file the `#?` notation is used:
 
-  #? stdtmpl(subsChar = '$', metaChar = '#')
-  #proc generateXML(name, age: string): string =
-  #  result = ""
-  <xml>
-    <name>$name</name>
-    <age>$age</age>
-  </xml>
+    #? stdtmpl(subsChar = '$', metaChar = '#')
+    #proc generateXML(name, age: string): string =
+    #  result = ""
+    <xml>
+      <name>$name</name>
+      <age>$age</age>
+    </xml>
 
 As the example shows, passing arguments to a filter can be done
 just like an ordinary procedure call with named or positional arguments. The
@@ -50,15 +50,15 @@ In your `main.nim`:
 Pipe operator
 =============
 
-Filters can be combined with the `|` pipe operator::
+Filters can be combined with the `|` pipe operator:
 
-  #? strip(startswith="<") | stdtmpl
-  #proc generateXML(name, age: string): string =
-  #  result = ""
-  <xml>
-    <name>$name</name>
-    <age>$age</age>
-  </xml>
+    #? strip(startswith="<") | stdtmpl
+    #proc generateXML(name, age: string): string =
+    #  result = ""
+    <xml>
+      <name>$name</name>
+      <age>$age</age>
+    </xml>
 
 
 Available filters
@@ -123,31 +123,31 @@ Parameters and their defaults:
 * `toString: string = "$"`
   : the operation that is applied to each expression
 
-Example::
+Example:
 
-  #? stdtmpl | standard
-  #proc generateHTMLPage(title, currentTab, content: string,
-  #                      tabs: openArray[string]): string =
-  #  result = ""
-  <head><title>$title</title></head>
-  <body>
-    <div id="menu">
-      <ul>
-    #for tab in items(tabs):
-      #if currentTab == tab:
-      <li><a id="selected"
-      #else:
-      <li><a
-      #end if
-      href="${tab}.html">$tab</a></li>
-    #end for
-      </ul>
-    </div>
-    <div id="content">
-      $content
-      A dollar: $$.
-    </div>
-  </body>
+    #? stdtmpl | standard
+    #proc generateHTMLPage(title, currentTab, content: string,
+    #                      tabs: openArray[string]): string =
+    #  result = ""
+    <head><title>$title</title></head>
+    <body>
+      <div id="menu">
+        <ul>
+      #for tab in items(tabs):
+        #if currentTab == tab:
+        <li><a id="selected"
+        #else:
+        <li><a
+        #end if
+        href="${tab}.html">$tab</a></li>
+      #end for
+        </ul>
+      </div>
+      <div id="content">
+        $content
+        A dollar: $$.
+      </div>
+    </body>
 
 The filter transforms this into:
 
@@ -183,36 +183,36 @@ whitespace) is converted to a string literal that is added to `result`.
 The substitution character introduces a Nim expression *e* within the
 string literal. *e* is converted to a string with the *toString* operation
 which defaults to `$`. For strong type checking, set `toString` to the
-empty string. *e* must match this PEG pattern::
+empty string. *e* must match this PEG pattern:
 
-  e <- [a-zA-Z\128-\255][a-zA-Z0-9\128-\255_.]* / '{' x '}'
-  x <- '{' x+ '}' / [^}]*
+    e <- [a-zA-Z\128-\255][a-zA-Z0-9\128-\255_.]* / '{' x '}'
+    x <- '{' x+ '}' / [^}]*
 
 To produce a single substitution character it has to be doubled: `$$`
 produces `$`.
 
 The template engine is quite flexible. It is easy to produce a procedure that
-writes the template code directly to a file::
+writes the template code directly to a file:
 
-  #? stdtmpl(emit="f.write") | standard
-  #proc writeHTMLPage(f: File, title, currentTab, content: string,
-  #                   tabs: openArray[string]) =
-  <head><title>$title</title></head>
-  <body>
-    <div id="menu">
-      <ul>
-    #for tab in items(tabs):
-      #if currentTab == tab:
-      <li><a id="selected"
-      #else:
-      <li><a
-      #end if
-      href="${tab}.html" title = "$title - $tab">$tab</a></li>
-    #end for
-      </ul>
-    </div>
-    <div id="content">
-      $content
-      A dollar: $$.
-    </div>
-  </body>
+    #? stdtmpl(emit="f.write") | standard
+    #proc writeHTMLPage(f: File, title, currentTab, content: string,
+    #                   tabs: openArray[string]) =
+    <head><title>$title</title></head>
+    <body>
+      <div id="menu">
+        <ul>
+      #for tab in items(tabs):
+        #if currentTab == tab:
+        <li><a id="selected"
+        #else:
+        <li><a
+        #end if
+        href="${tab}.html" title = "$title - $tab">$tab</a></li>
+      #end for
+        </ul>
+      </div>
+      <div id="content">
+        $content
+        A dollar: $$.
+      </div>
+    </body>