Markdown code blocks part 5 (#20236)

No logic was added, just a few more `*.md` files have been migrated.

doc/pegdocs.txt

@@ -175,8 +175,9 @@ The PEG parser implements this grammar (written in PEG syntax)::
 expression, identifiers are not interpreted as non-terminals, but are
 interpreted as verbatim string:
 
-.. code-block:: nim
+  ```nim
   abc =~ peg"abc" # is true
+  ```
 
 So it is not necessary to write ``peg" 'abc' "`` in the above example.
 
@@ -186,22 +187,25 @@ Examples
 
 Check if `s` matches Nim's "while" keyword:
 
-.. code-block:: nim
+  ```nim
   s =~ peg" y'while'"
+  ```
 
 Exchange (key, val)-pairs:
 
-.. code-block:: nim
+  ```nim
   "key: val; key2: val2".replacef(peg"{\ident} \s* ':' \s* {\ident}", "$2: $1")
+  ```
 
 Determine the ``#include``'ed files of a C file:
 
-.. code-block:: nim
+  ```nim
   for line in lines("myfile.c"):
     if line =~ peg"""s <- ws '#include' ws '"' {[^"]+} '"' ws
                      comment <- '/*' @ '*/' / '//' .*
                      ws <- (comment / \s+)* """:
       echo matches[0]
+  ```
 
 PEG vs regular expression
 -------------------------