restore Progress pages (bar/spinner/skeleton)

decided simplest approach was a mostly empty root `progress` page
that links to the other three.

packages/core/src/components/components.md

@@ -189,7 +189,7 @@ Check out the [React API docs](https://facebook.github.io/react/docs/react-api.h
 @page overlay
 @page popover
 @page portal
-<!--@page progress-bar-->
+@page progress
 @page slider
 @page table
 @page tabs

packages/core/src/components/progress/progress-bar.md

@@ -1,21 +1,17 @@
-@# Progress
-
-Blueprint provides two ways to indicate progress: a horizontal progress bar and a circular spinner.
-
-@## Progress bars
+@# Progress bars
 
 Progress bars can indicate determinate progress towards the completion of a task or an indeterminate
 loading state.
 
-@### CSS API
+@## CSS API
 
 Set the current progress of the bar via a `width` style rule on the inner `.pt-progress-meter`
 element. This is a very simple CSS-only component, and input validation for `width` values is
 limited: values above `100%` appear as 100% progress and values below `0%` appear as 0%.
 
 Omitting `width` will result in an "indeterminate" progress meter that fills the entire bar.
 
-@### JavaScript API
+@## JavaScript API
 
 The `ProgressBar` component is available in the __@blueprintjs/core__ package.
 Make sure to review the [general usage docs for JS components](#components.usage).

packages/core/src/components/progress/progress.md

@@ -0,0 +1,7 @@
+@# Progress
+
+Blueprint provides two ways to indicate progress: a horizontal progress bar and a circular spinner.
+
+@page progress-bar
+@page spinner
+@page skeleton

packages/core/src/components/skeleton/skeleton.md

@@ -1,8 +1,8 @@
-@## Skeletons
+@# Skeletons
 
 Skeletons allow you to show a loading state that mimics the shape of your yet-to-load content.
 
-@### CSS API
+@## CSS API
 
 Apply the class `.pt-skeleton` to elements that you would like to cover up with a loading animation.
 The skeleton inherits the dimensions of whatever element the class is applied to. This means that

packages/core/src/components/spinner/spinner.md

@@ -1,14 +1,14 @@
-@## Spinners
+@# Spinners
 
 Spinners indicate indeterminate progress.
 
-@### CSS API
+@## CSS API
 
 You can create spinners manually by inserting their whole markup into your HTML.
 Spinners created via markup use same modifier classes as the
 [React `Spinner` component](#components.progress.spinner.js).
 
-@### JavaScript API
+@## JavaScript API
 
 The `Spinner` component is available in the __@blueprintjs/core__ package.
 Make sure to review the [general usage docs for JS components](#components.usage).
@@ -35,7 +35,7 @@ are supported via the `className` prop.
 
 [msdn-css-svg]: https://developer.microsoft.com/en-us/microsoft-edge/platform/status/csstransitionsforsvgelements/?q=svg
 
-@#### SVG spinner
+@### SVG spinner
 
 Use the `SVGSpinner` component to render a spinner inside an SVG element.