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

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

* Markdown code blocks part 2; migrate Nim Manual

doc/backends.md

@@ -155,8 +155,7 @@ To wrap native code, take a look at the `c2nim tool <https://github.com/nim-lang
 with the process of scanning and transforming header files into a Nim
 interface.
 
-C invocation example
-~~~~~~~~~~~~~~~~~~~~
+### C invocation example
 
 Create a ``logic.c`` file with the following content:
 
@@ -194,8 +193,7 @@ could as well pass ``logic.o``) we could be passing switches to link any other
 static C library.
 
 
-JavaScript invocation example
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+### JavaScript invocation example
 
 Create a ``host.html`` file with the following content:
 
@@ -251,8 +249,7 @@ The name `NimMain` can be influenced via the `--nimMainPrefix:prefix` switch.
 Use `--nimMainPrefix:MyLib` and the function to call is named `MyLibNimMain`.
 
 
-Nim invocation example from C
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+### Nim invocation example from C
 
 Create a ``fib.nim`` file with the following content:
 
@@ -312,8 +309,7 @@ vary for each system. For instance, on Linux systems you will likely need to
 use `-ldl`:option: too to link in required dlopen functionality.
 
 
-Nim invocation example from JavaScript
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+### Nim invocation example from JavaScript
 
 Create a ``mhost.html`` file with the following content:
 

doc/manual_experimental.md

@@ -1463,8 +1463,7 @@ The operators `*`,  `**`, `|`, `~` have a special meaning in patterns
 if they are written in infix notation.
 
 
-The `|` operator
-~~~~~~~~~~~~~~~~~~
+### The `|` operator
 
 The `|` operator if used as infix operator creates an ordered choice:
 
@@ -1490,8 +1489,7 @@ semantics anyway. In fact, they can be deactivated with the `--patterns:off`:opt
 command line option or temporarily with the `patterns` pragma.
 
 
-The `{}` operator
-~~~~~~~~~~~~~~~~~~~
+### The `{}` operator
 
 A pattern expression can be bound to a pattern parameter via the `expr{param}`
 notation:
@@ -1504,8 +1502,7 @@ notation:
   echo a
 
 
-The `~` operator
-~~~~~~~~~~~~~~~~~~
+### The `~` operator
 
 The `~` operator is the 'not' operator in patterns:
 
@@ -1523,8 +1520,7 @@ The `~` operator is the 'not' operator in patterns:
   echo a
 
 
-The `*` operator
-~~~~~~~~~~~~~~~~~~
+### The `*` operator
 
 The `*` operator can *flatten* a nested binary expression like `a & b & c`
 to `&(a, b, c)`:
@@ -1559,8 +1555,7 @@ produces:
    `&&`("my", space & "awe", "some ", "concat")
 
 
-The `**` operator
-~~~~~~~~~~~~~~~~~~~
+### The `**` operator
 
 The `**` is much like the `*` operator, except that it gathers not only
 all the arguments, but also the matched operators in reverse polish notation:

doc/sets_fragment.txt

@@ -52,8 +52,7 @@ operation             meaning
 `excl(A, elem)`       same as `A = A - {elem}`
 ==================    ========================================================
 
-Bit fields
-~~~~~~~~~~
+### Bit fields
 
 Sets are often used to define a type for the *flags* of a procedure.
 This is a cleaner (and type safe) solution than defining integer