Migrate dev docs from piwik.org to developer.piwik.org #4279

The goal of this ticket is that the site developer.piwik.org contains all technical documentation about Piwik.

This is a follow up from developer.piwik.org project #4266

Here are the pages from piwik.org that can be moved to developer.piwik.org

• participate/development-metrics/ (can be removed)
• participate/user-interface/ http://developer.piwik/guides/core-development#user-interface
• participate/coding-standards/ http://developer.piwik.org/guides/core-development
• participate/contributing-with-git/ http://developer.piwik.org/guides/core-development

And these:

• docs/plugins/ (can be removed)
• docs/plugins/database-schema/ : http://developer.piwik/guides/introduction#database-schema (doc seems to be outdated)
• docs/plugins/framework-overview/ (can be removed)
• docs/plugins/functions/ (can be removed)
• docs/plugins/hooks/ (was already migrated)
• docs/plugins/plugin-development/framework-reference-security-checklist: http://developer.piwik.org/guides/plugins#security (Note: doc seems to be outdated)

See email for some links to piwik2 pages which are already converted to markdown.