Examples: use npx and yarn create to run create-next-app on examples

[jthegedus]

Use npx and yarn create to run create-next-app for each example in examples/.

This:

• requires no additional install from the user as these both come with their respective package managers (npx with Node ^8.x.x)
• skips an additional command, reducing the "getting-started" friction
• keeps global install of create-next-app up to date (not sure if npx actually pulls the latest)

  Installs create-<starter-kit-package> globally, or update the package to the latest version if it already exists - yarn docs

For those interested, this was done by:

1. removing the npm i -g ... line with:

for d in *
do
    ( cd $d && sed -i '/npm i -g create-next-app/d' README.md )
done

2. prepending npx to the execution command with VSCode
   Find: create-next-app --example
   Replace: npx create-next-app --example
3. adding the Yarn command with:

for d in *
do
    ( cd $d && sed -i "/npx create-next-app --example/a # or\nyarn create next-app --example $d $d-app" README.md )
done

This would have been easier had I learned sed properly prior to step 1. or 2. and just written a single command. Interesting nonetheless.

[jthegedus]

If this is not desired to be merged, please at least cherry-pick these commits:

• 0b0b18f - add bash to shell snippets
• 435b3a1 - change READMEs to use UPPERCASE

[timneutkens]

Looks great 👍 cc @fouad