Fix problem handling examples and interaction.

Unlike plain `racketblock`, `examples` and `interaction` are
"nested". As a result we emitted bad Markdown like:

    ```racket
    Examples:
    ```racket
    some-code
    ```
    ```

Markdown code blocks can't nest, so this needs to be:

    ```racket
    Examples:
    some-code
    ```

Also: Updated the unit test with examples of `examples` and
`interaction`.

original commit: a5f6686270239484a5e7b2e6864c9bd5627d937d

collects/scribble/markdown-render.rkt

@@ -88,6 +88,8 @@
         (define tick? (member (style-name (table-style i))
                               (list 'boxed "defmodule" "RktBlk")))
         (when tick?
+          (when (zero? (table-ticks-depth))
+            (displayln "```racket"))
           (table-ticks-depth (add1 (table-ticks-depth))))
         (define strs (map (lambda (flows)
                             (map (lambda (d)
@@ -111,8 +113,6 @@
                                     (apply max d (map string-length i)))))
                             (apply map list strs)))
         (define x-length (lambda (col) (if (eq? col 'cont) 0 (length col))))
-        (when tick?
-          (displayln (string-append "```racket")))
         (for/fold ([indent? #f]) ([row (in-list strs)])
           (let ([h (apply max 0 (map x-length row))])
             (let ([row* (for/list ([i (in-range h)])
@@ -133,8 +133,9 @@
                 #t)))
           #t)
         (when tick?
-          (displayln "```")
-          (table-ticks-depth (sub1 (table-ticks-depth)))))
+          (table-ticks-depth (sub1 (table-ticks-depth)))
+          (when (zero? (table-ticks-depth))
+            (displayln "```"))))
       null)
 
     (define/override (render-itemization i part ht)

collects/tests/scribble/markdown-docs/example.md

@@ -26,6 +26,8 @@ _Italic_. \_Just underlines\_.
 
 “Dobule quoted”. ‘Single quoted’.
 
+This should NOT be ‘code‘ in Markdown.
+
 Example of vebatim:
 
 `Hi, world.`  
@@ -64,6 +66,28 @@ Example of a defproc:
 Returns a new mutable string of length `k` where each position in the
 string is initialized with the character `char`
 
+Blah blah `(``or/c`` ``string?`` ``bytes?``)`.
+
+Example of Scribble `examples`:
+
+```racket
+Examples:      
+> (define x 0) 
+               
+> (displayln x)
+0              
+               
+```
+
+Example of Scribble `interaction`:
+
+```racket
+> (define x 0)
+              
+> x           
+0             
+```
+
 > Note: This is a note. Let’s make it long enough that the markdown output
 > will have to line-wrap, to make sure the > mark starts each line
 > properly.

collects/tests/scribble/markdown-docs/example.scrbl

@@ -1,7 +1,9 @@
-#lang scribble/doc
+#lang scribble/manual
 
-@(require scribble/manual
+@(require scribble/eval
           (for-label racket/base racket/contract racket/string))
+@(define my-eval (make-base-eval))
+@(my-eval '(require racket/base))
 
 @section{Section}
 
@@ -35,6 +37,8 @@ _Just underlines_.
 ``Dobule quoted''.
 `Single quoted'.
 
+This should NOT be `code` in Markdown.
+
 Example of vebatim:
 
 @verbatim{
@@ -71,8 +75,24 @@ Example of a defproc:
 Returns a new mutable string of length @racket[k] where each position in the
 string is initialized with the character @racket[char]
 
+Blah blah @racket[(or/c string? bytes?)].
+
 }
 
+Example of Scribble @racket[examples]:
+
+@examples[#:eval my-eval
+(define x 0)
+(displayln x)
+]
+
+Example of Scribble @racket[interaction]:
+
+@interaction[#:eval my-eval
+(define x 0)
+x
+]
+
 @margin-note{Note: This is a note. Let's make it long enough that the
 markdown output will have to line-wrap, to make sure the > mark starts
 each line properly.}