docs: add Mac LaunchAgent instructions

[cdaein]

From #999

Instructions for Mac LaunchAgent so the Shiori server will start at login automatically.

Note:
I tested on my Mac but it would be nice if someone else can also do a test to make sure.
Thanks!

[fmartingr]

Thank you for the contribution! Really appreciated.

I think is best if we put the macOS setup under it's own section, I've left a comment about that. Just don't forget to update the table of contents and we're good to go :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.66%. Comparing base (8c35a6b) to head (db841e6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1002      +/-   ##
==========================================
- Coverage   35.33%   33.66%   -1.67%     
==========================================
  Files          61       61              
  Lines        4078     5302    +1224     
==========================================
+ Hits         1441     1785     +344     
- Misses       2414     3294     +880     
  Partials      223      223              

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