feat(CategoryTheory/DiscreteCategory): binary sums and products of discrete categories

[robin-carlier]

We show that if C and D are discrete categories, then C ⊕ D and C × D are also discrete categories. This is stated both in terms of equivalences Discrete (J × K) ≌ Discrete J × Discrete K and Discrete (J ⊕ K) ≌ Discrete J ⊕ Discrete K, and in terms of the IsDiscrete typeclass.

We also implement a small refactor by creating a directory CategoryTheory/DiscreteCategory and moving CategoryTheory/DiscreteCategory.lean to CategoryTheory/DiscreteCategory/Basic.lean.

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I opted for the creation of a separate directory about DiscreteCategory so that one could still import DiscreteCategory.Basic but not necessarily the content of this PR, which imports CategoryTheory.Sums.Basic and CategoryTheory.Products.Basic.

[github-actions]

PR summary 65ee99958f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.DiscreteCategory	-314
Mathlib.CategoryTheory.Discrete.Basic	314
Mathlib.CategoryTheory.Discrete.SumsProducts (new file)	316

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Declarations diff

+ prod
+ productEquiv
+ sum
+ sumEquiv

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[callesonne]

Very clean, LGTM!

I also like the DiscreteCategory folder, but what about naming it just Discrete so its shorter? CategoryTheory.Discrete should still be an unambiguous import.

[robin-carlier]

DiscreteCategory directory has been renamed to Discrete.