Add Ruby Tutorial For Beginners (#3100)

* Add Ruby Tutorial For Beginners

Hi,
This adds a new free Ruby tutorial (HTML version), **optionally** visitors can download a PDF version of the book by entering an email address.

## What does this PR do?

Add Resource

### Description

Free Ruby tutorial / book for beginners in HTML format with multiple lessons, including installing Ruby, conditionals, loops & object-oriented programming.

### Why is this valuable

It's a complete Ruby tutorial for beginners that helps learn Ruby from scratch. Includes examples, clear explanations & exercises.

### How do we know it's really free?

I made it.

### For book lists, is it a book?

Yes.

### Checklist:

- [x] Not a duplicate
- [x] Included author(s) if appropriate
- [x] Lists are in alphabetical order
- [x] Needed indications added (PDF, access notes, under construction)

* Use Full Book Name

free-programming-books.md

@@ -2286,6 +2286,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Ruby Web Dev: The Other Way](https://leanpub.com/rwdtow/read) - Yevhen Kuzminov
 * [Rubyfu](https://rubyfu.net)
 * [The Bastards Book of Ruby](http://ruby.bastardsbook.com)
+* [The Definitive Ruby Tutorial For Complete Beginners](https://www.rubyguides.com/ruby-tutorial/) - Jesus Castello
 * [The Unofficial Ruby Usage Guide (at Google)](http://www.caliban.org/ruby/rubyguide.shtml) - Ian Macdonald
 * [Why's (Poignant) Guide to Ruby](http://poignant.guide)