🛑 Add Google Analytics to `mkdocs` site

[omerbensaadon]

Description:

BLOCKED BY #3776

Adding Google Analytics so we can track effectiveness of MkDocs migration.

See: knative/website#262 for tracking codes

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Story

As Omer
I want to see analytics about website visitors 
So that I can validate the changes we make are resulting in positive change 

Acceptance Criteria

• First Site Visit

GIVEN it is the first time I am accessing the Knative.dev site  
WHEN I load the homepage
THEN I see a small banner at the top of the page with the text detailed below 
AND I see an OK button
AND I see a "x" 

Copy:

We use analytics and cookies to understand site traffic. Information about your use of our site is shared with Google for that purpose. <a href= https://knative.dev/about-analytics-cookies/> Learn more</a>.

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• Omer accesses Google Analytics Dashboard

GIVEN I have access to the Knative.team
WHEN I access the Knative.team Google Analytics instance
THEN I see site data for the mkdocs site 

Known Issues and Considerations:

See: knative/website#262 for tracking codes

Who should probably pick up this issue?

• Existing Contributors
• New Contributors

[julz]

/assign