[3/?] Add ability to run code examples in the playground: Fetch tutorial snippet from URL by file name

.spelling-wordlist.txt

@@ -28,6 +28,7 @@ FFI
 filesystem
 finaliser
 finalisers
+fontawesome
 forgeable
 GC
 GDB

docs/generics/generics-and-reference-capabilities.md

@@ -50,7 +50,13 @@ That compiles and runs, so `ref` is valid now. The real test though is `iso`. Le
 This fails to compile. The first error is:
 
 ```error
---8<-- "generics-and-reference-capabilities-foo-iso-error-message.txt"
+main.pony:5:8: right side must be a subtype of left side
+    _c = c
+       ^
+    Info:
+    main.pony:4:17: String iso! is not a subtype of String iso: iso! is not a subtype of iso
+      new create(c: String iso) =>
+                ^
 ```
 
 The error is telling us that we are aliasing the `String iso` - The `!` in `iso!` means it is an alias of an existing `iso`. Looking at the code shows the problem:

docs/getting-started/hello-world.md

@@ -21,6 +21,8 @@ In your file, put the following code:
 --8<-- "hello-world-main.pony"
 ```
 
+[:fontawesome-solid-play: Run in playground](https://playground.ponylang.io/?snippet=hello-world-main.pony){ .md-button .md-button--primary target="_blank" }
+
 ## Compiling the program
 
 Now compile it:

mkdocs.yml

@@ -25,6 +25,10 @@ markdown_extensions:
   - pymdownx.snippets:
       base_path: ['code-samples']
       check_paths: true
+  - pymdownx.emoji:
+      emoji_index: !!python/name:material.extensions.emoji.twemoji
+      emoji_generator: !!python/name:material.extensions.emoji.to_svg
+  - attr_list
   - smarty
   - toc:
       permalink: true