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/plugins/developing/plugin.json.md

@@ -0,0 +1,28 @@
++++
+title = "plugin.json Schema"
+keywords = ["grafana", "plugins", "documentation"]
+type = "docs"
+[menu.docs]
+name = "plugin.json Schema"
+parent = "developing"
+weight = 6
++++
+
+# Plugin.json
+
+The plugin.json file is mandatory for all plugins. When Grafana starts it will scan the plugin folders and mount every folder that contains a plugin.json file unless the folder contains a subfolder named `dist`. In that case grafana will mount the `dist` folder instead.
+
+## Plugin JSON Schema
+
+| Property | Description |
+| ------------- |-------------|
+| id | unique name of the plugin - [conventions described in styleguide]({{< relref "code-styleguide.md" >}}) |
+| type | panel/datasource/app |
+| name | Human readable name of the plugin |
+| info.description | Description of plugin. Used for searching grafana net plugins |
+| info.author | |
+| info.keywords | plugin keywords. Used for search on grafana net|
+| info.logos | link to project logos |
+| info.version | project version of this commit. Must be semver |
+| dependencies.grafanaVersion | Required grafana backend version for this plugin |
+| dependencies.plugins | required plugins for this plugin. |