Add visitor tour with notebook links AB test

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/36779.

Tracking / event logs

Screenshots

BEFORE	AFTER
when all steps are completed/clicked

Recording

https://www.loom.com/share/f01275f9a44d40dba54c4b35332605e9

Test plan

• sg start dotcom
• Open https://sourcegraph.test:3443/search?feature-flag-key=tour-visitors-notebook&feature-flag-value=true as not signed in user
• Go through tour

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.