[JIRA] added new method get_issue_tree_recursive + docs + examples

[gkowalc]

I added a new method to the repo get_issue_tree_recursive which:

• method takes an issue key as input.
• Returns a list of dictionaries. Each dictionary represents a parent issue_key -> child_issue_key.
• Child_issue_key is either a subtask, or an inward link to another issue. I added warnings which specify limitations of the method - the user needs to have access to all linked issues and all linked issues must come from the same Jira instance (or from another instance if the user's API key also enables access to that instance).
• In addition to the method code, I have updated the docs and I have also added a sample use case.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 5.26316% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 34.18%. Comparing base (978fa3f) to head (08d4acb). Report is 1 commits behind head on master.

Files	Patch %	Lines
atlassian/jira.py	5.26%	18 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1352 +/- ## ========================================== - Coverage 34.25% 34.18% -0.07% ========================================== Files 46 46 Lines 8408 8427 +19 Branches 1178 1185 +7 ========================================== + Hits 2880 2881 +1 - Misses 5413 5431 +18 Partials 115 115 ```

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[gonchik]

@gkowalc thanks