docs: Document archival status of repository.

[mctavish]

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Fixes #1164 🦕

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8bb6588) 61.72% compared to head (ede2199) 61.72%.
Report is 2 commits behind head on main.

❗ Current head ede2199 differs from pull request most recent head 5b293fd. Consider uploading reports for the commit 5b293fd to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1191   +/-   ##
=======================================
  Coverage   61.72%   61.72%           
=======================================
  Files          17       17           
  Lines        1309     1309           
  Branches      257      257           
=======================================
  Hits          808      808           
  Misses        447      447           
  Partials       54       54           

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