docs(nav): give Plotting Prelude a title and put the four chapters in the sidebar

[breimanntools]

Two linked defects in Getting Started.

1. Plotting Prelude had no title. The first cell of tutorials/plotting_prelude.ipynb was a raw cell, not markdown. nbsphinx passes a raw cell through verbatim, so # Plotting Prelude never became a heading: the page had no title, rendered the hash as literal text glued onto the first paragraph, and appeared in the toctree as <no title>. It is the only one of the four Getting Started notebooks with this defect — the other three open with markdown.

2. The sidebar showed the wrong things. docs/source/getting_started.rst contained no toctree at all. The four notebooks were body-text :doc: links, which render as a list but wire nothing into navigation, so the sidebar fell back to the page's own sections (Quick start / The two APIs / Decision Map) instead of the four chapters.

They had to be fixed together: a toctree entry takes its target's title, so Plotting Prelude would have appeared as <no title> in the sidebar.

Verification (full build, inspecting the generated HTML)

<title>  : "Plotting Prelude — AAanalysis"   (was <no title>)
first h1 : "Plotting Prelude"                (was none)
literal "# Plotting Prelude" in body: gone

Getting Started sidebar children:
  A minimal CPP analysis      -> generated/tutorial0_minimal.html
  Quick start with AAanalysis -> generated/tutorial1_quick_start.html
  Slow start with AAanalysis  -> generated/tutorial1_slow_start.html
  Plotting Prelude            -> generated/plotting_prelude.html

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.39%. Comparing base (3c11c90) to head (b0334c6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #584   +/-   ##
=======================================
  Coverage   95.39%   95.39%           
=======================================
  Files         222      222           
  Lines       23387    23387           
  Branches     4073     4073           
=======================================
  Hits        22309    22309           
  Misses        631      631           
  Partials      447      447           

Components	Coverage Δ
cpp_core	95.97% <ø> (ø)

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