Update documentation of offline installation

[isc-pbarton]

Resolves #447

Installing from a tar.gz file is now simpler in IPM.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.34%. Comparing base (749c582) to head (c1a9fac).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #878      +/-   ##
==========================================
+ Coverage   49.72%   50.34%   +0.61%     
==========================================
  Files          24       24              
  Lines        3298     3629     +331     
==========================================
+ Hits         1640     1827     +187     
- Misses       1658     1802     +144     

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