feat(Combinatorics/SimpleGraph/Girth): relate girth and egirth

[SnirBroshi]

Also add le_girth to match le_egirth, which is the only egirth lemma that's missing a girth counterpart.

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[github-actions]

PR summary d306712143

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 (regex)

+ coe_girth_eq_egirth_iff
+ coe_girth_le_egirth
+ girth_eq_iff
+ girth_eq_iff_of_ne_zero
+ girth_eq_iff_of_not_isAcyclic
+ girth_eq_toNat_egirth
+ le_girth
+ le_girth_iff_coe_le_egirth

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit d306712).

• +8 new declarations
• −0 removed declarations

+SimpleGraph.coe_girth_eq_egirth_iff
+SimpleGraph.coe_girth_le_egirth
+SimpleGraph.girth_eq_iff
+SimpleGraph.girth_eq_iff_of_ne_zero
+SimpleGraph.girth_eq_iff_of_not_isAcyclic
+SimpleGraph.girth_eq_toNat_egirth
+SimpleGraph.le_girth
+SimpleGraph.le_girth_iff_coe_le_egirth

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit d306712143
Reference commit d3716e6d2e

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/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).