Update Plugins page in Documentation

Plugins page created for 5.0 has grown significantly over the year. Proposed to:

1. Move plugin writing instructions to Development guidelines
2. Move plugin setup steps, currently described in Zabbix agent 2 templates operation to Plugins page (since not everyone using a plugin may use it in conjunction with a template); on the Agent 2 template operation page add a backlink to the Plugins page.
3. Move generic Plugin configuration subsection higher on the page and the chart with plugin descriptions lower.