Docs: add meta tag

[warriordog]

The documentation site needs <meta /> tags on each page. This should be put into the main layout (mainLayout.html) with a parameter to set the meta value. There should always a meta tag on each page, so some kind of placeholder should be used if a value is not provided. Another slightly more complex but cleaner solution is to have a list of "common" meta keys for all pages, and then combine that with any additional keys passed in through parameters.

Parts of this task:

• Add meta tag to page template
• Add "common" meta tags (look at github sidebar and package.json for ideas)
• Add page-specific meta tags