Docs for developing plugins (#7475)

* docs(plugins): new developing plugins section

Creates new section called Developing Plugins in
the plugin section of the docs.

1. Some changes to the Development guide page
2. Converted defaults/editor mode blog post
to new page
3. Converted snapshots blog post to new page
4. Adds new code styleguide page
5. Updates to apps and datasources pages
6. Adds plugin.json schema

* docs(links): fixes broken links

Fixes broken links to other pages as
well as broken image links.

docs/sources/reference/playlist.md

@@ -16,7 +16,7 @@ Since Grafana automatically scales Dashboards to any resolution they're perfect
 
 ## Creating a Playlist
 
-{{< docs-imagebox img="img/docs/v3/playlist.png" max-width="25rem" >}}
+{{< docs-imagebox img="/img/docs/v3/playlist.png" max-width="25rem" >}}
 
 The Playlist feature can be accessed from Grafana's sidemenu, in the Dashboard submenu.