Recurrence layer

[CarloLucibello]

Add a Recurrence layer that takes a cell and makes it able to process an entire sequence as once.
The name is taken from the corresponding Lux layer
Also updated the guide with how to stack recurrent layers and add dropout in between.
Close #1040 close #2452
Part of #2514

[codecov]

Codecov Report

Attention: Patch coverage is 7.14286% with 13 lines in your changes missing coverage. Please review.

Project coverage is 31.95%. Comparing base (2bbd8b3) to head (c4c582d).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layers/recurrent.jl	7.14%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2549      +/-   ##
==========================================
- Coverage   31.95%   31.95%   -0.01%     
==========================================
  Files          34       34              
  Lines        1987     1984       -3     
==========================================
- Hits          635      634       -1     
+ Misses       1352     1350       -2     

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