Document ignore_case string functions

[aljazerzen]

🔍 Problem

• The docs do not describe the new ignore_case argument on string functions.
• There is no reference page yet for equals.

🛠️ Solution

• Document ignore_case on starts_with, ends_with, contains,
  replace, and split.
• Add a new equals reference page.
• Add examples for both ASCII and Unicode case-folding behavior.

💬 Review

• Focus on the signatures and examples for the new case-insensitive behavior.
• replace_regex and split_regex are intentionally unchanged here as well.

_{⚙️ Code PR: tenzir/tenzir#6378
🎫 References TNZ-696}

[github-actions]

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