Append .template to every template file

[mtfishman]

This is friendlier for things like formatters since the .jl template files can't be parsed as Julia code.

[github-actions]

Your PR no longer requires formatting changes. Thank you for your contribution!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.53%. Comparing base (ee10e9f) to head (aabd1d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   85.00%   89.53%   +4.53%     
==========================================
  Files           1        1              
  Lines          80       86       +6     
==========================================
+ Hits           68       77       +9     
+ Misses         12        9       -3     

Flag	Coverage Δ
docs	83.95% <95.00%> (+14.61%)	⬆️

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