docs: add Tableau websites to README (#54)

* Add links to Demo Sites

* Apply suggestions from code review

---------

Co-authored-by: Mitchell Hanberg <[email protected]>

Author: andyl

Diff for: README.md

@@ -30,9 +30,13 @@ end
 
 Documentation can be found at <https://hexdocs.pm/tableau>.
 
-## Demo
+## Built with Tableau
 
-For a real world demo of Tableau, you can see [www.elixir-tools.dev](https://www.elixir-tools.dev) ([source code](https://github.com/elixir-tools/elixir-tools.dev)).
+| Site                                                 | Template    | Source                                                                            |
+|------------------------------------------------------|-------------|-----------------------------------------------------------------------------------|
+| [www.elixir-tools.dev](https://www.elixir-tools.dev) | [Temple](https://github.com/mhanberg/temple) | [elixir-tools/elixir-tools.dev](https://github.com/elixir-tools/elixir-tools.dev) |
+| [pdx.su](https://pdx.su)                             | [Temple](https://github.com/mhanberg/temple) | [paradox460/pdx.su](https://github.com/paradox460/pdx.su)                         |
+| HEEx Demo                                            | [HEEx](https://hexdocs.pm/phoenix_live_view/Phoenix.Component.html#sigil_H/2)   | [mhanberg/tableau_demo_heex](https://github.com/mhanberg/tableau_demo_heex)       |
 
 ## Getting Started