Add XML documentation for Android.Telephony.Euicc

The `Android.Telephony.Euicc` namespace contains **108** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Android.Telephony.Euicc/**' --glob 'ns-Android.Telephony.Euicc.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **122** to **108** (reduced by **14**). Related documentation PRs: [#446](https://github.com/dotnet/android-api-docs/pull/446), [#545](https://github.com/dotnet/android-api-docs/pull/545), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided PowerShell ripgrep command from the repository root to list the 108 placeholders under docs\xml for Android.Telephony.Euicc, excluding docs\xml\index.xml. Read the surrounding XML and related documentation entries before updating each placeholder. Done means the scan no longer reports those 108 `To be added` entries and the documentation remains valid.

Vom Indexierungsmodell aus dem Issue-Text verfasst.