add link to Pyfive GitHub repository on doc landing page

[valeriupredoi]

Description

As suggested in #183 for better back and forth navigation.

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.33%. Comparing base (78fa31f) to head (1b1e862).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #185   +/-   ##
=======================================
  Coverage   77.33%   77.33%           
=======================================
  Files          15       15           
  Lines        2974     2974           
  Branches      477      477           
=======================================
  Hits         2300     2300           
  Misses        555      555           
  Partials      119      119           

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