add additional procedural macro links (exercism#1249)

I've been implementing a procedural macro this week and found these
links quite useful to learn the skill.

As an aside, I wonder about the order of links in `links.json`.
I'm curious if the specification declares normative ordering. I've been
informally ordering them using beginning with more practically focused
topics and ending on the more theoretical ones. i.e. a bias toward
practice-based learning materials.

concepts/macros-procedural/links.json

@@ -3,6 +3,14 @@
     "url": "https://blog.rust-lang.org/2018/12/21/Procedural-Macros-in-Rust-2018.html",
     "description": "Wonderfully accessible introductory post by Alex Crichton."
   },
+  {
+    "url": "https://doc.rust-lang.org/book/ch19-06-macros.html#procedural-macros-for-generating-code-from-attributes",
+    "description": "Basic introduction to writing procedural macros"
+  },
+  {
+    "url": "https://github.com/dtolnay/proc-macro-workshop",
+    "description": "Self-directed practice for learning to write basic and advanced procedural macros."
+  },
   {
     "url": "https://doc.rust-lang.org/reference/procedural-macros.html",
     "description": "Introduction to procedural macros from The Rust Reference."