Feature/prismic webhook

[clari182]

• closes Update Prismic Webhook instructions #3306

How to test

• Publish a document on Clara's prismic
• Check Clara's repo actions. It should trigger two jobs, one for staging and one for production

Before merging

• Add GITHUB_TOKEN to Netlify (other new vars have been added)
• Add PRISMIC_SECRET to github project settings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.13%. Comparing base (8c95bef) to head (c1d4d96).

Additional details and impacted files

@@           Coverage Diff            @@
##           staging    #3331   +/-   ##
========================================
  Coverage    96.13%   96.13%           
========================================
  Files           65       65           
  Lines        13020    13020           
  Branches       223      322   +99     
========================================
  Hits         12517    12517           
+ Misses         503      489   -14     
- Partials         0       14   +14     

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