add monodocs redirect banner

[cosmicBboy]

This PR adds a banner at the top of the flytesnacks docs (the flytekit readthedocs project), pointing users to the main flyte docs site, which will contain the monodocs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9c45cb2) 85.75% compared to head (2146580) 85.75%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2091   +/-   ##
=======================================
  Coverage   85.75%   85.75%           
=======================================
  Files         313      313           
  Lines       23433    23433           
  Branches     3511     3511           
=======================================
  Hits        20095    20095           
  Misses       2730     2730           
  Partials      608      608           

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