docs: reorganize settings guidance

[oesteban]

Summary

• restructure the installation settings section around TemplateFlowClient configuration
• document cache location, DataLad mode, and S3 origin workflows with CLI and Python examples
• explain autoupdate, timeout, and layout refresh behaviour using the new cache metadata descriptors

Testing

• not run (documentation only)

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https://chatgpt.com/codex/tasks/task_e_68f70abb7ef483308539f3441b2f0143

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.62%. Comparing base (7718e6a) to head (0591437).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #152   +/-   ##
=======================================
  Coverage   77.62%   77.62%           
=======================================
  Files           9        9           
  Lines         447      447           
  Branches       82       82           
=======================================
  Hits          347      347           
  Misses         63       63           
  Partials       37       37           

Flag	Coverage Δ
api,datalad	51.90% <ø> (ø)
api,dls3	27.29% <ø> (ø)
api,s3	57.27% <ø> (ø)
config	74.04% <ø> (ø)

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