New docs for 3.0.0

[jennifer-shehane]

Dashboard doc:

• update screenshots, etc to show new tabs.
• Explain new 'Specs' and 'Insights' tabs
• Remove mention of 'output' / 'screenshots' 'videos' tabs - now in modals

New 'recommendations' for how to speed up tests

• maybe this should be in best practices??
• recommend having small spec files

Specs running in isolation

• not sure where to mention this

New kitchen-sink tests

• need to update screenshots of seeded files
• update folder structure https://docs.cypress.io/guides/core-concepts/writing-and-organizing-tests.html#Folder-Structure
• update any mention of example_spec

Update config with new spec globbing

• --spec now accepts globs

we will direct / document common solutions for things like aggregating multiple reporter output (JSON, JUnit, etc)
document how to use multiple reporters
document reducing the default minimum of snapshots from 50, down to 5.
We'll modify the workflow and documentation to suggest iterating on a spec and not trying to run all tests from the Test Runner.