[Docs] add proportionsZTest to docs

[Blargian]

Closes #1996 as part of the functions project.

Adds:

• proportionsZTest

Changelog category (leave one):

• Documentation (changelog entry is not required)

[robot-clickhouse-ci-2]

This is an automated comment for commit 9234bea with description of existing statuses. It's updated for the latest CI running

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Check name	Description	Status
A Sync	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	⏳ pending
Mergeable Check	Checks if all other necessary checks are successful	❌ failure

Successful checks

Check name	Description	Status
CI running	A meta-check that indicates the running CI. Normally, it's in success or pending state. The failed status indicates some problems with the PR	✅ success
Docs check	Builds and tests the documentation	✅ success
PR Check	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	✅ success
Style check	Runs a set of checks to keep the code style clean. If some of tests failed, see the related log from the report	✅ success