docs: update the timezone page for the fixes from the timezone EPIC

[andygrove]

Which issue does this PR close?

No issue closes. Part of #6335.

Rationale for this change

#6337 added timezones.md before the fixes from the same audit landed. Four of them changed behavior the page describes as current: #6347 (CASE and COALESCE no longer panic on a timestamp branch with another label), #6351 (session timezone IDs are normalized before they reach native code), #6353 (a warning when the native and JVM timezone databases differ), and #6349 (the native CSV scan falls back for timestamps outside UTC). The page still says the CASE casts panic, describes a timeZoneId.getOrElse("UTC") fallback that no longer exists on main, and says native code cannot parse GMT+8, Z or PST.

What changes are included in this PR?

• "The invariant": CASE, COALESCE and IF now relabel a mismatched timestamp branch (coerce_branch in case_when.rs), so they move from the list of things that depend on the label to the places where a wrong label hides.
• "How the session timezone reaches native code": serdes go through CometTimeZone.nativeId, an expression with no timezone gets "UTC", and native code reports an empty timezone as an error (require_timezone) instead of asserting.
• "Parsing timezone IDs": which forms nativeId rewrites, and what happens to an offset with seconds such as +05:45:30.
• "Timezone rules": the warning NativeBase logs when the tzdata versions differ, with a pointer to the user guide section.
• "Scans": the native CSV scan's fallback.
• "The codegen dispatcher" and "Guidelines": a timezone native code cannot express goes through the dispatcher, and timezones reach native code only through nativeId.
• "Testing timezone-sensitive code": session_timezone_ids.sql, avoiding dates that recent tzdata releases changed, and checking that an expression ran natively rather than in the dispatcher, since a dispatched expression takes its whole subtree with it. It drops "put it in a CASE", which no longer exposes a wrong label.

#6340, which points the PR review skills at this page, is being rebased to match.

How are these changes tested?

This is a docs-only change. prettier --check passes. A local Sphinx build gives the same warnings with and without the change. I checked each statement against the code on main: CometTimeZone.scala, case_when.rs, utils.rs, NativeBase.java, CometScanRule.scala and CometScalaUDF.scala.

[sunchao]

Summary

• Prior state and problem: The timezone contributor guide described behavior superseded by existing fixes.
• Design approach: Updates the documentation to match current timezone normalization, conditional-branch coercion, database-version warnings, and CSV fallback behavior.
• Correctness / compatibility analysis: The revised claims match the referenced Comet implementation. Checked Spark timezone resolution and CSV timestamp parsing against sources for supported versions 3.4.3, 3.5.9, 4.0.4, 4.1.3, and 4.2.0.
• Key design decisions: Retains the timestamp-label invariant and points contributors to existing helpers. Introduces no runtime overhead or additional abstractions.
• Implementation sketch: Changes only docs/source/contributor-guide/timezones.md, including guidance for verifying native execution and detecting dispatched subtrees.
• Behavioral changes worth calling out: Documents existing fixes without changing execution. Compared with latest release branch branch-1.1, which does not contain this page. This PR itself introduces no runtime behavior change.
• Suggested improvements: None at the P1/P2 threshold. No introduced P1/P2 issues found within this review.

Reviewed the entire diff from ba08acd815d5fd15f75ce9f314715778b9161cf1 to 2e8c9d9e74685bc2d19268406814ba58d5c32fb5. The PR is not a draft. Snapshot and live discussion checks found no existing reviews, comments, or review threads.

Routed skills: review-comet-pr. No sibling area skill applies to this documentation-only diff.

Exact-head CI: 7 checks passed and 16 were skipped, with no failures. Preflight passed Markdown formatting and repository checks. JVM/native builds, Spark SQL, and Iceberg suites were skipped.

Validation: Source comparisons, git diff --check, and all four relative Markdown link targets passed. No runtime suites were run. Sphinx is unavailable locally, so the rendered documentation was not built. The project worktree remains unchanged.