feat: add scripts to test if there are keys in references.bib with non-ASCII characters

[acmepjz]

Currently doc-gen4 does not support non-ASCII characters as reference keys in reference.bib. What's worse, it will stops processing that file silently. This leads to various mathlib4 docs reference page errors.

This PR adds scripts to test if there are keys in references.bib with non-ASCII characters. A demonstration is at https://github.com/leanprover-community/mathlib4/actions/runs/13759366997/job/38472088509.

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[github-actions]

PR summary bed8be186b

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

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.

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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).

[grunweg]

LGTM, but I'd like somebody else to double-check.

[acmepjz]

LGTM, but I'd like somebody else to double-check.

It's best if the script can output a diff with non-ASCII characters in key replaced by corresponding ASCII characters. But I haven't figured out how to do it using bibtool. From bibtool documentation seems that there is a macro iso2tex.