* 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.

* fix backendSrv request cancelling

* revert imports

* formatting

Fix typo

Add image url to telegram alert notifier

If public image url of alert is exist add it to the telegram message

fix(packaging): fixed issue in deb package post install script that made it think upgrade was happening, fixes #7403