adding producer examples

[DanielePalaia]

On the README at the moment we don't explain all the method we can use to send messages but we just give an example for send_with_confirm which is the slowest way to send messages (and likely not the most popular one used).

I created 3 examples: send_async, batch_send and send_with_confirm and modified the README accordingly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.74%. Comparing base (30df99f) to head (b8cdf53).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   89.44%   89.74%   +0.30%     
==========================================
  Files          78       78              
  Lines        6662     6662              
==========================================
+ Hits         5959     5979      +20     
+ Misses        703      683      -20     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.