[Docs] Pull together example sites & plugins for docs

[DavidWells]

To improve the docs we need to start collecting & highlighting example repos & plugins they might have.

Starting a list here:

• https://github.com/DavidWells/netlify-build-exit-build-early
• https://github.com/DavidWells/netlify-build-programatic-plugin-usage
• https://github.com/talves/netlify-plugin-debug
• TODO add more 😃

Plugins list

• https://github.com/netlify/plugins + https://github.com/netlify/plugins/blob/master/plugins.json
• WIP internal plugins https://github.com/netlify-labs?utf8=%E2%9C%93&q=netlify-plugin&type=&language=
• https://www.notion.so/netlify/e017077d66e94626b02fe0e96a02e8b5?v=4a0873c0694f4f0dbba8f3fe70169db2
• https://www.notion.so/netlify/e017077d66e94626b02fe0e96a02e8b5?v=4a0873c0694f4f0dbba8f3fe70169db2

Use cases / examples

• https://github.com/netlify/build/issues?q=is%3Aissue+is%3Aopen+label%3A%22plugin+idea%22
• https://www.notion.so/netlify/Build-Plugin-Examples-Use-Cases-3269e7eb7bb24453b497b04898d91166

[ehmicky]

I believe this is done now.