docs: add source code link in nav bar

[heitorlessa]

Issue #, if available: #220

Description of changes:

This creates a new button on side bar pointing to the GitHub repository.

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

Breaking change checklist

RFC issue #:

• Migration process documented
• Implement warnings (if it can live side by side)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[pankajagrawal16]

[codecov-io]

Codecov Report

Merging #223 (e972257) into develop (704eba7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #223   +/-   ##
========================================
  Coverage    99.88%   99.88%           
========================================
  Files           70       70           
  Lines         2600     2600           
  Branches       109      109           
========================================
  Hits          2597     2597           
  Misses           3        3           

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