Example notebook: kernel ridge regression (#320)

Extended tutorial-style kernel ridge regression example that starts from linear regression, shows how to get to kernel ridge regression, and how to use KernelFunctions.jl kernels instead of manually adding feature vectors.

Also changes the `preprocess` in literate.jl to filter out `##` lines to delimit code blocks in VSCode.

Author: st--

docs/literate.jl

@@ -13,8 +13,8 @@ Pkg.activate(EXAMPLEPATH)
 Pkg.instantiate()
 using Literate: Literate
 
-# Add link to nbviewer below the first heading of level 1
 function preprocess(content)
+    # Add link to nbviewer below the first heading of level 1
     sub = SubstitutionString(
         """
 #md # ```@meta
@@ -35,7 +35,12 @@ function preprocess(content)
 #
         """,
     )
-    return replace(content, r"^# # [^\n]*"m => sub; count=1)
+    content = replace(content, r"^# # [^\n]*"m => sub; count=1)
+
+    # remove VSCode `##` block delimiter lines
+    content = replace(content, r"^##$."ms => "")
+
+    return content
 end
 
 # Convert to markdown and notebook