docs: add nightly CDN disclaimer for EmulatorJS netplay

[Copilot]

When netplay is enabled, EmulatorJS loads assets (including localization files) from the nightly CDN (https://cdn.emulatorjs.org/nightly/...) rather than local/bundled assets — an implementation detail that has caused user confusion and 404 errors in the past.

Changes

• Platforms-and-Players/EmulatorJS-Player.md — Added a note admonition at the end of the Netplay section explaining the nightly CDN behavior and potential temporary issues.
• Getting-Started/Configuration-File.md — Added the same note admonition after the netplay YAML block in the config reference.

Both notes read:

When netplay is enabled, EmulatorJS loads some assets (including localization files) from the nightly CDN (https://cdn.emulatorjs.org/nightly/...). This differs from stable mode, which uses local/bundled assets. Occasional temporary issues (e.g., 404 errors or untranslated UI elements) can occur if the nightly CDN has mismatches, but these usually resolve with the next EmulatorJS stable release integrated into RomM.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Other] Documentation should mention using EmulatorJS nightly CDN when netplay is enabled.</issue_title>
<issue_description>### Describe the request / problem
The RomM documentation (EmulatorJS player section and config.yml reference) explains how to enable netplay. However, it doesn't mention that enabling netplay causes EmulatorJS to load certain assets, such as localization files, from the nightly CDN (https://cdn.emulatorjs.org/nightly/...) instead of the bundled/local assets used when netplay is disabled.

This is an implementation detail that isn't obvious to users. It led to confusion and 404 errors on localization files in the past (see closed issue rommapp/romm#2855), due to filename mismatches/changes on the EmulatorJS nightly CDN (e.g., from lang_country.json to lang.json or similar). Even after fixes or stable releases, users might still encounter temporary issues, console errors, or missing translations without knowing the cause.

Suggested change

A simple disclaimer to help avoid future confusion and issues such as:

"When netplay is enabled, EmulatorJS loads some assets (including localization files) from the nightly CDN (https://cdn.emulatorjs.org/nightly/...). This differs from stable mode, which uses local/bundled assets. Occasional temporary issues (e.g., 404 errors or untranslated UI elements) can occur if the nightly CDN has mismatches, but these usually resolve with the next EmulatorJS stable release integrated into RomM."
</issue_description>

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

• Fixes [Other] Documentation should mention using EmulatorJS nightly CDN when netplay is enabled. #63

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