Design Adjustments

I just realized my commits were rather large, sorry about this.

Here are the **styling/coding** changes:
• The majority of the colours were stripped from the docs, it was getting too crazy-town. Created new variables and/or adjusted them where appropriate
• The typography was adjusted to ensure better readability, this means colour, line-height, spacing with p tags, headings and typeface
• The spacing was adjusted in all breakpoints through the entire document and I introduced better separators for each section
• Went through the document for section consistency. There were some links that weren't styled as links, some heading styles that weren't used anywhere else, etc.
• All code/examples were taken out of the tab style that I was using in favour of just one-after-another. This makes it far easier to find the content you're looking for, as you no longer have to look for it!
• Reduced size of icons in the header
• Moved prettyprint into a partial (as discussed with Carson) and un-minified it so we can adjust it in the future if we want to
• Added new styles where they were needed (docs and sources now have a call-out style)
• Added a custom highlight (this was unnecessary but I like green; YOLO)

Here are the **content** changes:
• I ditched the secondary header that sat underneath the primary header to reduce the noise and to make the information easier to read and parse
• Because of ^, some of the sentences were removed completely if they weren't adding to the point of the paragraph
• The section titled "Getting Started" was rearranged slightly. I moved the "Demo Store" section up to replace "Selling Shopify Themes" section because it has more to do with working with Timber
• Selling Shopify Themes is at the bottom of this section now as it's more of an additional sell for Partners than it is related to "Getting Started"
• I reorganized the Typography section because the layout didn't add to what we were trying to show
• I may have added or removed a few words here and there for grammatical reasons

There may be some further adjustments to styles as soon as I create this request, but it should be done in about ten-twenty minutes.

@cshold @pauldpritchard @stevebosworth