Add debug and file handling guides for CheerpJ JNLP Runner

[rijulshrestha]

Adds the JNLP Runner versions of the two Applet Runner guides already live under cheerpj-applet-runner/guides.

New pages

• /docs/cheerpj-jnlp-runner/guides/debug-builds: checks to confirm the extension is working, how to turn on Debug Mode, and how to collect the console log and a HAR file
• /docs/cheerpj-jnlp-runner/guides/file-handling: uploading through the title-bar icon or the side panel (/files/uploads/), and downloading through /files/downloads/

Differences from the Applet Runner guides

The text matches the Applet Runner guides except where the JNLP Runner works differently:

• Debug Mode is turned on in the popup's Advanced Settings → General → Debug Mode (Slow), not with a toggle in the popup
• Files are uploaded from the side panel (the JNLP popup has no Upload button)
• The JNLP-specific checks replace the Applet-only ones (per-domain toggle, page contains an applet)
• DevTools and HAR steps follow the JNLP launch flow
• GitHub links point to cheerpj-jnlp-runner/issues

Screenshots

• Added popup_advanced_settings.png and debug_mode_toggle.png
• Reused jnlp_sidepanel.png for the side-panel upload step

The screenshot's colour looks a bit off on my screen compared to the one in applet runners docs. Could you confirm if this is the case.

[epanholz]

Left a few comments for some minor changes, rest looks good.