Docs: Created the Wasmtime Markdown Parser Example (#3193)

* Finished the Markdown Parser Example for Wasmtime

* Made requested changes

* Tiny change to explanation of `--dir` CLI arg

* Add `bash` annotations to shell script code blocks

* Trying to fix the markdown example bug

* Figured out rustdoc, and what needed to be done

* Made requested changes

Co-authored-by: Till Schneidereit <till@tillschneidereit.net>

docs/rust_wasi_markdown_parser/src/main.rs

@@ -0,0 +1,46 @@
+// Import our CLI parsing libraries (And PathBuf for reading paths)
+extern crate structopt;
+
+use structopt::StructOpt;
+use std::path::PathBuf;
+
+// Import our markdown parser library, crate
+extern crate pulldown_cmark;
+
+use pulldown_cmark::{html, Parser};
+
+// Import from the standard library, to allow reading from the file system
+use std::fs;
+
+// Define our CLI options using structopt
+#[derive(StructOpt)]
+#[structopt(name = "rust_wasi_markdown_parser", about = "Markdown to HTML renderer CLI, written with Rust & WASI")]
+pub struct Options {
+    /// The markdown file to render
+    #[structopt(parse(from_os_str))]
+    filename: PathBuf,
+}
+
+// Our entrypoint into our WASI module
+fn main() {
+
+    // Get the passed CLI options
+    let options = Options::from_args();
+
+    // Read the markdown file into a string
+    let contents = fs::read_to_string(options.filename)
+        .expect("Something went wrong reading the file");
+
+    // Run our parsing function to get back an HTML string
+    let result = render_markdown(contents);
+
+    // Print out the resulting HTML to standard out
+    println!("{}", result);
+}
+
+pub fn render_markdown(markdown: String) -> String {
+    let mut html_buf = String::new();
+    let parser = Parser::new(&markdown[..]);
+    html::push_html(&mut html_buf, parser);
+    html_buf
+}