Add link to optunahub-registry

[y0z]

Motivation & Description of the changes

• Add links to optunahub-registry and its documentation from the optunahub docs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (3ea405d) to head (66c3d4f).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   88.80%   88.80%           
=======================================
  Files           6        6           
  Lines         134      134           
=======================================
  Hits          119      119           
  Misses         15       15           

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[not522]

@HideakiImamura Could you review this PR?

[nabenabe0928]

LGTM!