Add Docs for Reusable Filters

[karandatwani92]

What are reusable Filters? How to setup reusable Filters?

Add info to following page:

• https://backpackforlaravel.com/docs/7.x-dev/crud-filters

[pxpm]

Hey @karandatwani92 docs for reusable filters are here: https://backpackforlaravel.com/docs/7.x-dev/crud-filters#use-filters-on-custom-admin-panel-pages

[karandatwani92]

I gave it a read from readers prospective and felt something disconnected:

• How ReportsController@ index will be called?
• How and where returned view('admin.reports') will be rendered.

I feel, completing the example would help readers understand and use it quicker.