Add/update docs for 6.0 changes

[chrisbreiding]

• Renaming pluginsFile to backgroundFile. Also restructure the API docs around plugins to fall in line with the new nomenclature.
• cy.task handler merging
• New background events
• Background events that mirror driver events
• Write a migration guide for the breaking changes

[jennifer-shehane]

Notes from recent discussion over changes:

[chrisbreiding]

For each event, either in the table or in its own page, should communicate whether or not and how the event handler blocks.

• synchronously?
• promise-aware?
• do errors bubble up?