Update plugin installation documentation

[mouligno]

The current documentation regarding plugin management is very light, we should improve that part in order to have the proper followup guide for every kind of installation (Docker/aws/zip...)

[plsstephenson]

Refer to the current documentation for cross-referencing etc.:

• https://docs.gravitee.io/apim/3.x/apim_overview_plugins.html
• https://docs.gravitee.io/apim/3.x/apim_devguide_plugins.html
• https://docs.gravitee.io/apim/3.x/apim_policies_overview.html