feat(NumberTheory/Chebyshev): express the Chebyshev theta function in terms of the prime counting function

[antisubnoous]

This is a sibling theorem to Chebyshev.primeCounting_eq_theta_div_log_add_integral.

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This is my first mathlib4 commit. I am working through some exercises and results in Apostol's ANT book as a means to practice Lean, and this result seemed like an opportunity to practice the mechanics of contributing to Mathlib because the new theorem mirrors closely the sibling theorem above (both appear in Apostol Theorem 4.3).

[github-actions]

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[github-actions]

PR summary 8dbc02da01

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

+ theta_eq_primeCounting_mul_log_sub_integral

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 scripts/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).