feat(Real): add bound y * exp(-y) ≤ exp(-1)

[jackabatc]

Summary

• Add Real.mul_exp_neg_le_exp_neg_one : y * exp (-y) ≤ exp (-1).
• Proof is a short consequence of Real.add_one_le_exp.
• Placed in Mathlib/Analysis/SpecialFunctions/ExpBounds.lean.

[github-actions]

PR summary be04ab25ca

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Analysis.SpecialFunctions.ExpBounds (new file)	1658

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Declarations diff

+ mul_exp_neg_le_exp_neg_one

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).

[vihdzp]

I don't think this theorem deserves its own file. Can you put it in the import instead?