Add precompile signatures to Markdown to reduce latency. (#55715)

Fixes #55706 that is seemingly a 4472x regression, not just 16x (was my
first guess, based on CondaPkg, also fixes or greatly mitigates
JuliaPy/CondaPkg.jl#145), and large part of 3x
regression for PythonCall.

---------

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

Author: 2 people

stdlib/Markdown/src/Markdown.jl

@@ -122,4 +122,25 @@ import Base.Docs: catdoc
 
 catdoc(md::MD...) = MD(md...)
 
+if Base.generating_output()
+    # workload to reduce latency
+    md"""
+    # H1
+    ## H2
+    ### H3
+    **bold text**
+    *italicized text*
+    > blockquote
+    1. First item
+    2. Second item
+    3. Third item
+    - First item
+    - Second item
+    - Third item
+    `code`
+    Horizontal Rule
+    ---
+    """
+end
+
 end