Precompile Markdown parsing AND printing (#55752)

---------

Co-authored-by: Kristoffer <[email protected]>
Co-authored-by: Ian Butterworth <[email protected]>

Author: 3 people

stdlib/Markdown/src/Markdown.jl

@@ -138,23 +138,35 @@ catdoc(md::MD...) = MD(md...)
 
 if Base.generating_output()
     # workload to reduce latency
-    md"""
+    show(devnull, MIME("text/plain"), md"""
     # H1
     ## H2
     ### H3
+    #### H4
+    ##### H5
+    ###### H6
     **bold text**
     *italicized text*
+    ***bold and italicized text***
     > blockquote
     1. First item
     2. Second item
     3. Third item
     - First item
     - Second item
     - Third item
+        - Indented item
     `code`
     Horizontal Rule
     ---
-    """
+    **[Duck Duck Go](https://duckduckgo.com)**
+    <https://www.markdownguide.org>
+    <[email protected]>
+    ![The San Juan Mountains are beautiful!](/assets/images/san-juan-mountains.jpg "San Juan Mountains")
+
+    H~2~O
+    X^2^
+    """)
 end
 
 end