Add new_magic.rst: guide for creating custom magics

[blink1073]

Summary

Fixes #282

• Adds docs/source/new_magic.rst addressing possible / how to define a new magic ? #282 — documents how to write, install, and distribute custom magics for any MetaKernel-based kernel
• Covers line/cell magic authoring, the @option decorator, user-local install (~/.ipython/metakernel/magics/), %install_magic, bundling with a kernel package, output helpers, docstring conventions, and IPython compatibility
• Links the new page from docs/index.rst
• Adds test_local_magics_dir_loaded to verify that magics placed in ~/.ipython/metakernel/magics/ are discovered at kernel startup

Test plan

• just test tests/test_metakernel.py::test_local_magics_dir_loaded passes
• just docs builds without warnings

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.88%. Comparing base (3e230e3) to head (ab7edff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #351   +/-   ##
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  Coverage   90.88%   90.88%           
=======================================
  Files          51       51           
  Lines        2854     2854           
  Branches      396      396           
=======================================
  Hits         2594     2594           
  Misses        181      181           
  Partials       79       79           

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