TCP T4: add the native client documentation outline

[alex-clickhouse]

Summary

Stacked on #609.

• replace the C# overview with a short guide to choosing HTTP or native TCP
• move the current HTTP documentation unchanged, apart from page metadata, to its own page
• add a heading-only outline for the native TCP documentation
• add both protocol pages to the C# navigation

Verification

• ClickHouse docs-builder scoped Mintlify validation passed
• site-wide internal link and anchor check passed with 0 errors
• configured zizmor high-severity gate passed with no findings
• TCP net9 test suite: 4,097 passed, 5 skipped

The solution-wide net9 run also encountered the net10-only integration-test project and four unrelated HTTP GetSchema failures caused by an existing Dynamic(max_types=2) table on the shared test server.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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