Make documentation compatible with Ansible Galaxy

[Donien]

To be able to display the Collection's documentation on Ansible Galaxy (and by proxy by using ansible-doc) we should change the location of certain pieces of documentation, merge and add some files.

Even though most of the modules and plugins within this Collection are meant for internal use, ansible-doc still finds them but can't show any documentation.
We should at least add a basic description.

• Collection README
  • Avoid links or use absolute ones since Galaxy can't seem to handle relative ones
• Roles README
  • icinga2
  • icingadb
  • icingadb_redis
  • icingaweb2
  • monitoring_plugins
  • repos
• Modules / Plugins
  • icinga2_apilistener
  • icinga2_apiuser
  • icinga2_checkcommand
  • icinga2_checkercomponent
  • icinga2_dependency
  • icinga2_elasticsearchwriter
  • icinga2_endpoint
  • icinga2_eventcommand
  • icinga2_externalcommandlistener
  • icinga2_filelogger
  • icinga2_gelfwriter
  • icinga2_graphitewriter
  • icinga2_host
  • icinga2_hostgroup
  • icinga2_icingaapplication
  • icinga2_icingadb
  • icinga2_idomysqlconnection
  • icinga2_idopgsqlconnection
  • icinga2_influxdb2writer
  • icinga2_influxdbwriter
  • icinga2_livestatuslistener
  • icinga2_notification
  • icinga2_notificationcommand
  • icinga2_notificationcomponent
  • icinga2_opentsdbwriter
  • icinga2_perfdatawriter
  • icinga2_scheduleddowntime
  • icinga2_service
  • icinga2_servicegroup
  • icinga2_sysloglogger
  • icinga2_timeperiod
  • icinga2_user
  • icinga2_usergroup
  • icinga2_zone

Ref:

• Documentaion / Getting started not visible on Ansible Galaxy WebUI #274

[Donien]

Some progress:
main...doc/321-ansible-galaxy