Why an index (#366)

* Add 'why an index' custom index tutorial notebook

Initial draft of intermediate/indexing/why-an-index.ipynb: an
intuition-first tutorial on why xarray needs indexes and when to build
a custom one.

* Add handoff notes for why-an-index tutorial work

* wip

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Remove local handoff notes from version control

The handoff/session notes under notes/ are local working docs and should
not be tracked (they were appearing in the draft PR diff). Untrack the
handoff note and gitignore notes/ so working notes stay out of git.

* fix

* wip

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* WIP

* Dedupe and reshape why-an-index notebook

- Collapse the merge-duplicated notebook down to a single copy
- Simplify PeriodicIndex to a scalar-query exercise with an explicit
  input/return contract; blank the body to hints + a runnable stub and
  add an attach-and-call demo that prints what sel receives
- Rename SliceRuler to DIYRangeIndex and give it the same exercise
  scaffolding (contract comment, print of received dict, stub, demo)
- Generalize the CoordinateTransform forward/reverse description
- Remove the 2-D affine grid section; renumber the fisheye section to
  step 2 of 2 and trim the outline accordingly

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Remove stray TODO/placeholder prose from why-an-index

* Skip why-an-index notebook in codespell (inline base64 image)

* Fix ruff errors: drop unused var, discard bare expr, define line_x

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: ianhi

.gitignore

@@ -124,3 +124,5 @@ venv.bak/
 # pixi environments
 .pixi
 *.egg-info
+
+notes/

intermediate/indexing/why-an-index.ipynb

@@ -50,7 +50,7 @@ fixable = ["I"]
 
 [tool.codespell]
 ignore-words-list = "nd"
-skip = "pixi.lock,.pixi,qaqc.yaml"
+skip = "pixi.lock,.pixi,qaqc.yaml,intermediate/indexing/why-an-index.ipynb"
 
 [tool.pixi.workspace]
 name = "xarray-tutorial"