[Confluence] added new method get_space_export + docs + examples

[gkowalc]

Added a new method to export spaces from Confluence Cloud (I don't have DC license available, so the method might not work properly with confluence on-prem) as requested in #1458.

As mentioned in the example file and comments in the code, this is an experimental method that implements a workaround to Atlassian REST API space export limitation - https://jira.atlassian.com/browse/CONFCLOUD-40457

I run my tests on relatively small space sizes it is possible that atl_token might expire before export completes (and as a result execution of while loop will be stuck) so feel free to run your tests on your production confluence cloud instances and in case of any issues let the maintainers know in the ticket #1458

[codecov]

Codecov Report

Attention: Patch coverage is 2.00000% with 49 lines in your changes missing coverage. Please review.

Project coverage is 34.16%. Comparing base (fb9c354) to head (b2df92c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
atlassian/confluence.py	2.00%	49 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1466      +/-   ##
==========================================
- Coverage   34.34%   34.16%   -0.19%     
==========================================
  Files          46       46              
  Lines        8664     8714      +50     
  Branches     1319     1327       +8     
==========================================
+ Hits         2976     2977       +1     
- Misses       5567     5616      +49     
  Partials      121      121              

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