Introduction to HTML presentation files

First set of slides with HTML content for days 1 and 2
Update introduction_to_html.md

Fixed missing embedded code tag
Update useful_tags.md

added a better description to title tag
Update useful_tags.md

Added link to HTML language reference
Added HTML exercise 1

Exercise 1 provides unformatted text and an image showing how the layout
should appear. Students should markup the text so that it can be laid
out as shown in the image.
Added sample html file with TOC
Update best_practices.md

Added link to brackets tutorial. improved ada formatting