Developer notes for release v3.0.0 preparation

[florence-bockting]

Description

This PR adds developer notes to support the preparation of the loo v3.0.0 release. All files are in notes/, which is not part of the package:

• loo-v3-cleanup-todos.md: a checklist of open TODOs, for example missing links and placeholders. Each item names the PR it belongs to and what it waits for.
• loo-v3-documentation.md: an overview of the tutorials, case studies, glossary and other documents that exist, are new or are planned. It sets one name for each document, so that we refer to each document in the same way.
• loo-v3-related-prs-issues.md: an overview of the PRs and issues that relate to the v3.0.0 release and later releases.

We update these notes while we prepare the release.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.50%. Comparing base (2636314) to head (b15a95f).
⚠️ Report is 3 commits behind head on loo-v3.0.0.

Additional details and impacted files

@@             Coverage Diff             @@
##           loo-v3.0.0     #402   +/-   ##
===========================================
  Coverage       92.50%   92.50%           
===========================================
  Files              31       31           
  Lines            3107     3107           
===========================================
  Hits             2874     2874           
  Misses            233      233           

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if b15a95f is merged into loo-v3.0.0:

• ✔️loo_function: 1.45s -> 1.45s [-0.34%, +0.11%]
• ✔️loo_matrix: 1.38s -> 1.38s [-0.24%, +0.14%]
  Further explanation regarding interpretation and methodology can be found in the documentation.