docs: add react example code

[vinckr]

Adds the react example to ReactJS under
http://ory.sh/kratos/docs/next/self-service/flows/user-login#code-examples-for-nodejs-reactjs-go-
and the other flows

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #2185 (12c5b7b) into master (1d0482c) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2185      +/-   ##
==========================================
- Coverage   75.33%   75.32%   -0.02%     
==========================================
  Files         294      294              
  Lines       15651    15651              
==========================================
- Hits        11791    11789       -2     
- Misses       3029     3030       +1     
- Partials      831      832       +1     

Impacted Files	Coverage Δ
persistence/sql/persister_courier.go	85.00% <0.00%> (-3.34%)	⬇️

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