Improvements to intermediate/regex

Improvements to the Introduction and Simple Patterns sections of intermediate/regex.

Main changes:
* Fixed the links to other sections and the Glossary.
* Added files containing the 'evil' data to data/evil.
* Created some exercises for the Simple Patterns section.
* Corrected a few typos and made some paragraphs clearer.
* Split up the Simple Patterns section into subsections.
* Headers have been added to each Markdown file, and the intermediate/regex section has been added to the Makefile.
* Added an index page.
Please avoid empty lines between "in" and "out" blocks.