Add XML documentation for Java.Time.Zone

The `Java.Time.Zone` namespace contains **105** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Java.Time.Zone/**' --glob 'ns-Java.Time.Zone.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **169** to **105** (reduced by **64**). Related documentation PRs: [#485](https://github.com/dotnet/android-api-docs/pull/485), [#533](https://github.com/dotnet/android-api-docs/pull/533), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to enumerate the 105 placeholders under Java.Time.Zone and inspect the matching source XML files. Replace the relevant “To be added” documentation, using the Java.Time.Zone API context, and rerun the scan until no placeholders remain in that namespace.

Scritto dal modello di indicizzazione a partire dal testo della issue.