Update tests and docs to use `RSpec.describe` as the outer block

In RSpec 4.0 its monkey patching will be disabled by default, which means the describe, context, and other methods won't be available globally anymore.

• https://relishapp.com/rspec/rspec-core/docs/configuration/zero-monkey-patching-mode
• https://rspec.info/blog/2013/07/the-plan-for-rspec-3/#zero-monkey-patching-mode

The unit/integration spec files have already been updated: gitlab-org/gitlab#220018 (closed)

Some of the docs have been updated, and the rest will be too: gitlab-org/gitlab#220218 (closed)

We use an outer context block in the E2E tests. We should change that to RSpec.describe, to be consistent with all the other spec files, and we should update our docs (including the beginner's guide).