Reorganize docs for easier deploys

With the current docs directory setup, I'm making too many mistakes and have to manually address path changes and directory moves on deploy. This makes for a frustrating experience developing locally and shipping releases. With this PR, we're basically back to the same setup from v3—duplicating the dist directory into our docs directory. Not the most ideal, but very straightforward for me as the release manager.

site/_data/nav.yml

@@ -0,0 +1,86 @@
+- title: Getting started
+  pages:
+    - title: Introduction
+    - title: Download
+    - title: Contents
+    - title: Browsers & devices
+    - title: JavaScript
+    - title: Theming
+    - title: Build tools
+    # - title: Best practices # TODO: Write this content
+    - title: Webpack
+    - title: Accessibility
+
+- title: Layout
+  pages:
+    - title: Overview
+    - title: Grid
+    - title: Media object
+    - title: Utilities for layout
+
+- title: Content
+  pages:
+    - title: Reboot
+    - title: Typography
+    - title: Code
+    - title: Images
+    - title: Tables
+    - title: Figures
+
+- title: Components
+  pages:
+    - title: Alerts
+    - title: Badge
+    - title: Breadcrumb
+    - title: Buttons
+    - title: Button group
+    - title: Card
+    - title: Carousel
+    - title: Collapse
+    - title: Dropdowns
+    - title: Forms
+    - title: Input group
+    - title: Jumbotron
+    - title: List group
+    - title: Modal
+    - title: Navs
+    - title: Navbar
+    - title: Pagination
+    - title: Popovers
+    - title: Progress
+    - title: Scrollspy
+    - title: Tooltips
+
+- title: Utilities
+  pages:
+    - title: Borders
+    - title: Clearfix
+    - title: Close icon
+    - title: Colors
+    - title: Display
+    - title: Embed
+    - title: Flex
+    - title: Float
+    - title: Image replacement
+    - title: Position
+    - title: Screenreaders
+    - title: Shadows
+    - title: Sizing
+    - title: Spacing
+    - title: Text
+    - title: Vertical align
+    - title: Visibility
+
+- title: Extend
+  pages:
+    - title: Approach
+    - title: Icons
+
+- title: Migration
+
+- title: About
+  pages:
+    - title: Overview
+    - title: Brand
+    - title: License
+    - title: Translations