[examples] Add a Gatsby example

[oliviertassinari]

Closes #9777

[magicmark]

@oliviertassinari Seems to not be building for me with a fresh install:

╔═ mark@mark: /var/folders/68/4brvn_sn33l8hdvkwsd5wnr80000gn/T/tmp.UMSrtAGKNJ
╚═ ♪ curl https://codeload.github.com/mui-org/material-ui/tar.gz/v1-beta | tar -xz --strip=2  material-ui-1-beta/examples/gatsby

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1824k    0 1824k    0     0  1837k      0 --:--:-- --:--:-- --:--:-- 1837k

╔═ mark@mark: /var/folders/68/4brvn_sn33l8hdvkwsd5wnr80000gn/T/tmp.UMSrtAGKNJ/gatsby
╚═ ♪ yarn
yarn install v1.3.2
info No lockfile found.
[1/4] 🔍  Resolving packages...
warning gatsby > babel-preset-es2015@6.24.1: 🙌  Thanks for using Babel: we recommend using babel-preset-env now: please read babeljs.io/env to update!
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
warning "gatsby > postcss-cssnext@2.11.0" has unmet peer dependency "caniuse-db@^1.0.30000652".
warning "material-ui > react-scrollbar-size@2.0.2" has unmet peer dependency "lodash@^4.17.0".
[4/4] 📃  Building fresh packages...
[1/3] ⠂ fsevents
[1/3] ⠄ fsevents
[-/3] ⠠ waiting...
[-/3] ⠐ waiting...
[2/3] ⠐ uws
[-/3] ⢀ waiting...
[-/3] ⠠ waiting...
[2/3] ⠠ uws
success Saved lockfile.
✨  Done in 24.25s.

╔═ mark@mark: /var/folders/68/4brvn_sn33l8hdvkwsd5wnr80000gn/T/tmp.UMSrtAGKNJ/gatsby
╚═ ♪ yarn run develop
yarn run v1.3.2
$ gatsby develop
success delete html files from previous builds — 0.005 s
success open and validate gatsby-config.js — 0.004 s
success copy gatsby files — 0.015 s
success onPreBootstrap — 0.005 s
success source and transform nodes — 0.014 s
success building schema — 0.060 s
success createLayouts — 0.005 s
success createPages — 0.005 s
success createPagesStatefully — 0.021 s
success onPreExtractQueries — 0.000 s
success update schema — 0.037 s
success extract queries from components — 0.053 s
success run graphql queries — 0.021 s
success write out page data — 0.001 s
success write out redirect data — 0.000 s
success onPostBootstrap — 0.000 s

info bootstrap finished - 2.562 s

error There was an error compiling the html.js component for the development server.

See our docs page on debugging HTML builds for help https://goo.gl/yL9lND

  78 |  */
  79 | function decomposeColor(color) {
> 80 |   if (color.charAt(0) === '#') {
     | ^
  81 |     return decomposeColor(convertHexToRGB(color));
  82 |   }
  83 |


  WebpackError: Cannot read property 'charAt' of undefined

  - colorManipulator.js:80 decomposeColor
    ~/material-ui/styles/colorManipulator.js:80:1

  - colorManipulator.js:226 lighten
    ~/material-ui/styles/colorManipulator.js:226:1

  - createPalette.js:144 createPalette
    ~/material-ui/styles/createPalette.js:144:1

  - createMuiTheme.js:71 createMuiTheme
    ~/material-ui/styles/createMuiTheme.js:71:1

  - getPageContext.js:10 Object.defineProperty.value
    src/getPageContext.js:10:30

  - gatsby-ssr.js:6 Object.defineProperty.value
    gatsby-ssr.js:6:1

  - api-runner-ssr.js:2 Object.<anonymous>
    .cache/api-runner-ssr.js:2:15


error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

related to #9590 / #8075 maybe?

[magicmark]

Ah this is probably due to the changes here
https://github.com/mui-org/material-ui/releases/tag/v1.0.0-beta.28

The following diff fixes this, I'll send a PR

diff --git a/src/getPageContext.js b/src/getPageContext.js
index ad98484..70b7891 100644
--- a/src/getPageContext.js
+++ b/src/getPageContext.js
@@ -9,8 +9,16 @@ import green from 'material-ui/colors/green';
 // It's optional.
 const theme = createMuiTheme({
   palette: {
-    primary: purple,
-    secondary: green,
+    primary: {
+      light: purple[300],
+      main: purple[500],
+      dark: purple[700],
+    },
+    secondary: {
+      light: green[300],
+      main: green[500],
+      dark: green[700],
+    }
   },
 });