docs: Add regexp_extract function and Date64 support for date_format

[Copilot]

Documents new functions and extended type support from GreptimeDB PR #7107.

Changes

• Added regexp_extract function - Extracts first substring matching a regex pattern, returns NULL on no match

  SELECT regexp_extract('version 1.2.3', '\d+\.\d+\.\d+');
  -- Returns: 1.2.3
  
  SELECT regexp_extract('no match', '\d+');
  -- Returns: NULL

• Updated to_char/date_format documentation - Now explicitly documents Date64 support alongside existing Date32 and Timestamp types

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update docs for feat: adds format, regex_extract function and more type tests</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#7107</issue_description>

<agent_instructions>Update the function doc based on the original database pull request.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for feat: adds format, regex_extract function and more type tests #2177

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[killme2008]

LGTM

[fengjiachun]

LGTM