Add three CC-BY-SA licensed, Ada's PDF resources

[ghost]

What does this PR do?

Add Resource(s)

For resources

Ada free ebooks. Mainly introduction to Ada 2012, subset of Ada SPARK (for formal verification), and Ada SPARK for MISRA C developers.

Why is this valuable (or not)

There are not many free Ada ebooks lying around the internet, especially the good ones like these.

How do we know it's really free?

Explicit CC-BY-SA license notice in the book page after the content indices, usually before the preface and introduction page.

For book lists, is it a book?

Yes, ebooks.

Checklist:

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

[eshellman]

Do you think it might be better to link to pages like: https://learn.adacore.com/courses/SPARK_for_the_MISRA_C_Developer/ ?

[eshellman]

Also, these are super additions!

[ghost]

@eshellman Do I need to put these somewhere else other than this file if I link to the pages directly instead?

[eshellman]

no, same place. but leave off the pdf indication, since it's pdf and html.

[ghost]

@eshellman Alright. I've converted them to use the pages instead.

[eshellman]

Great, thanks.