Add grader header and footer templates

[jgwerner]

• Add default header template with source/header.ipynb
• Add default footer template with source/footer.ipynb
• Update readme

[codecov]

Codecov Report

Merging #305 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #305   +/-   ##
=======================================
  Coverage   80.29%   80.29%           
=======================================
  Files          21       21           
  Lines        1304     1304           
=======================================
  Hits         1047     1047           
  Misses        257      257           

Impacted Files	Coverage Δ
src/illumidesk/setup_course/constants.py	100.00% <ø> (ø)

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[netoisc]

LGTM