Demo documentation reorganization

[olivialynn]

Inspired by:
(a) how confused I felt when trying to find a good place to place the "Margin Catalogs" notebook, and
(b) I was curious to see how Claude Code would perform at a larger scale (that said--I've read everything it wrote and coded here! As I've revised and iterated on things as necessary, I stand behind the changes proposed in this PR)

Mostly opening this PR for the sake of seeing the RTD preview

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.15%. Comparing base (fd2c9a1) to head (5182d3e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1395   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files          49       49           
  Lines        3147     3147           
=======================================
  Hits         3026     3026           
  Misses        121      121           

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[github-actions]

Before [fd2c9a1]	After [3d36a15]	Ratio	Benchmark (Parameter)
1.09±0.01s	1.12±0.01s	1.02	benchmarks.time_create_midsize_catalog
374±6ms	381±6ms	1.02	benchmarks.time_open_many_columns_default
28.7±0.2ms	29.0±0.1ms	1.01	benchmarks.time_box_filter_on_partition
7.27±0.02s	7.33±0.02s	1.01	benchmarks.time_create_large_catalog
187±3ms	188±4ms	1	benchmarks.time_kdtree_crossmatch
10.1±0.04s	10.1±0.08s	1	benchmarks.time_lazy_crossmatch_many_columns_overlapping_suffixes
3.83±0.04s	3.81±0.04s	1	benchmarks.time_open_many_columns_all
168±3ms	168±1ms	1	benchmarks.time_open_many_columns_list
9.90±0.02s	9.89±0.04s	1	benchmarks.time_save_big_catalog
10.2±0.01s	10.0±0.04s	0.99	benchmarks.time_lazy_crossmatch_many_columns_all_suffixes

Click here to view all benchmarks.