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/dashboard.md

@@ -66,11 +66,11 @@ Each field in the dashboard JSON is explained below with its usage:
 | **editable** | whether a dashboard is editable or not |
 | **hideControls** | whether row controls on the left in green are hidden or not |
 | **graphTooltip** | TODO |
-| **rows** | row metadata, see [rows section](/docs/sources/reference/dashboard.md/#rows) for details |
+| **rows** | row metadata, see [rows section](#rows) for details |
 | **time** | time range for dashboard, i.e. last 6 hours, last 7 days, etc |
-| **timepicker** | timepicker metadata, see [timepicker section](/docs/sources/reference/dashboard.md/#timepicker) for details |
-| **templating** | templating metadata, see [templating section](/docs/sources/reference/dashboard.md/#templating) for details |
-| **annotations** | annotations metadata, see [annotations section](/docs/sources/reference/dashboard.md/#annotations) for details |
+| **timepicker** | timepicker metadata, see [timepicker section](#timepicker) for details |
+| **templating** | templating metadata, see [templating section](#templating) for details |
+| **annotations** | annotations metadata, see [annotations section](#annotations) for details |
 | **schemaVersion** | TODO |
 | **version** | TODO |
 | **links** | TODO |
@@ -79,7 +79,7 @@ Each field in the dashboard JSON is explained below with its usage:
 
 `rows` field consists of an array of JSON object representing each row in a dashboard, such as shown below:
 
-```
+```json
  "rows": [
     {
       "collapse": false,
@@ -105,14 +105,14 @@ Usage of the fields is explained below:
 | **collapse** | whether row is collapsed or not |
 | **editable** | whether a row is editable or not |
 | **height** | height of the row in pixels |
-| **panels** | panels metadata, see [panels section](/docs/sources/reference/dashboard.md/#panels) for details |
+| **panels** | panels metadata, see [panels section](#panels) for details |
 | **title** | title of row |
 
 #### panels
 
 Panels are the building blocks a dashboard. It consists of datasource queries, type of graphs, aliases, etc. Panel JSON consists of an array of JSON objects, each representing a different panel in a row. Most of the fields are common for all panels but some fields depends on the panel type. Following is an example of panel JSON representing a `graph` panel type:
 
-```
+```json
 "panels": [
         {
           "aliasColors": {},
@@ -276,7 +276,7 @@ Usage of each field is explained below:
 
 Description: TODO
 
-```
+```json
 "timepicker": {
     "collapse": false,
     "enable": true,
@@ -330,7 +330,7 @@ Usage of the fields is explained below:
 
 `templating` fields contains array of template variables with their saved values along with some other metadata, for example:
 
-```
+```json
  "templating": {
     "enable": true,
     "list": [