Update banner image guidance in dashboards doc (#9461)
Update banner image guidance in dashboards doc (#9461)

* Update banner image guidance with more clarity about relative paths

* Clarify copy around creating a web-ui PR for banner images

Co-authored-by: Jorie Helwig <22afce13a6b6dcd4613ae18f1ac68ca4776b5c5b@datadoghq.com>

Co-authored-by: Jorie Helwig <22afce13a6b6dcd4613ae18f1ac68ca4776b5c5b@datadoghq.com>
DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core