Create "Base" documentation

[wlla]

We want to create a category of documentation that's not an atom, molecule, organism, or template. The guidelines we want to capture here can have more global impact. Examples might include:

• Color contrast: our requirements for contrast ratios for text and ui
• Logic for creating page titles
• Requirements for lang
• Focus indicators or general guidelines on focus style
• We might even add colors in general to this section rather classify it as an atom

In addition to being reference during active development, this is a gift to our future selves and colleagues so we don't have déjà vu. This also means we don't need to rewrite things like our color contrast requirements for each component.

[wlla]

Would also be good to have a page around forms and one about search (which would also depend on principles in the forms section). Included would be pattern of always enclosing search terms in quotes (including empty strings). Originally developed in DR-762.