Enable/rollout "Getting started tour for visitors" on dotcom mode

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/27177 Closes https://github.com/sourcegraph/sourcegraph/issues/30258

Note: also cleans up concluded "search notebook examples" experiment https://github.com/sourcegraph/sourcegraph/issues/23096

Description

This PR:

• Enables "Getting started tour for visitors" (previously was in AB test) on dotcom mode
• Renames "onboarding tour" prefix to "getting started tour" to avoid confusion with "search onboarding tour"
• Removes old/unused search/notebook examples from LoggedOutHomePage

Test plan

• SOURCEGRAPH_API_URL=https://sourcegraph.com sg start web-standalone
• Open home page as not signed in user
• Check that "Getting Started" section is there and other "search onboarding tour" is not shown up
• Sign in and check that "getting started" section doesn't appear

Follow-up

Delete "getting-started-tour" feature flag