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 34.74%. Comparing base (6b6d5f3) to head (c43aa5b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1002   +/-   ##
=======================================
  Coverage   34.74%   34.74%           
=======================================
  Files          61       61           
  Lines        5322     5322           
=======================================
  Hits         1849     1849           
  Misses       3249     3249           
  Partials      224      224           

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[fmartingr]

This looks good! Thank you for your contribution. :)