Help the user use the extension

[fregante]

Error messages and notifications are useless at the moment: They tell you what's wrong, but not how to fix it.

• [x] Add a "get started" document to point people to
• [x] Customize the extension errors to point to this document ("No field found" and "No editor found")
• [x] In the options page, add link to this document and to the testing ground/demo page.
• [x] Point errors to their exact troubleshooting parts
• [x] Add welcome page to open on install https://github.com/fregante/ghosttext.fregante.com/issues/2