Updated documentation (faq and usage)

[sarthakpati]

Fixes #N.A.

Proposed Changes

• added an faq about where performance metrics are stored
• added usage notes about anonymizer

Checklist

• I have read the CONTRIBUTING guide
• My PR is based from the current GaNDLF master
• Non-breaking change (would not break existing functionality): please provide as many details as possible for any breaking change
• Function/class source code documentation added/updated
• Code has been blacked for style consistency
• If applicable, version information has been updated in GANDLF/version.py
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file
• Usage documentation has been updated, if appropriate
• History has been updated, if appropriate
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation

[codecov]

Codecov Report

Merging #294 (224eef6) into master (0c3f52c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   87.71%   87.71%           
=======================================
  Files          83       83           
  Lines        4266     4266           
=======================================
  Hits         3742     3742           
  Misses        524      524           

Flag	Coverage Δ
unittests	87.71% <ø> (ø)

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