feat(Rat): (q₁ + q₂).den ∣ q₁.den.lcm q₂.den

[jhanschoo]

add a useful theorem Rat.add_den_dvd_lcm that claims that the denominator of the addition of two rational numbers divides the LCM of the denominators of its terms. A stronger result than Rat.add_den_dvd that is already present, which still remains useful in many cases.

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[github-actions]

PR summary d58b085839

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

+ add_den_dvd_lcm

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).

[Ruben-VandeVelde]

Thanks! Some suggestions to match mathlib style more closely below

[Ruben-VandeVelde]

Please remove the awaiting-author label when you've addressed these comments