[CDN] Add video delivery troubleshooting and slicing guidance

[apih9000]

What changed

Adds a video-specific troubleshooting guide and strengthens the existing Video CDN delivery docs, based on an analysis of recurring video delivery support cases (CDN delivery and origin-side issues). The goal is to let customers self-diagnose the most common problems and tell CDN faults apart from origin faults.

Files changed

• cdn/video-cdn/troubleshoot-video-delivery.mdx — new article. Covers viewer-facing issues (403/410 secure-token failures on CGNAT/mobile, buffering from a cold cache, and 404s when live MPEG-DASH players request segments ahead of the live edge) plus origin issues that look like CDN faults (504 on newly published files, 5xx during live peaks, and content the CDN can't cache due to origin Cache-Control headers). Each case has an explicit action list.
• cdn/video-cdn/improve-video-delivery-speed.mdx — added VOD/Live cache-hit-ratio benchmarks (split per delivery type), a cache-warming-over-time example, a "Slice very large single-file videos" note, a "Check response codes and origin response time" section (Reports → Statistics: Response classes, Upstream response time), and supporting screenshots.
• cdn/cdn-resource-options/optimize-large-file-delivery.mdx — added an "Example of large file slicing" section (10 MB chunked range fetch) and a note that HLS/DASH don't need slicing.
• cdn/cdn-resource-options/cache/specify-cache-lifetime-on-a-cdn-resource-or-origin.mdx — cache lifetime guidance updates supporting the above.
• cdn/getting-started/create-a-cdn-resource/create-a-cdn-resource-for-video-streaming.mdx — related setup updates.
• docs.json — registered the new troubleshooting page in the Video CDN nav group.

TODO before publishing

• Confirm the live MPEG-DASH 404 fix steps (<UTCTiming>, suggestedPresentationDelay, timeShiftBufferDepth) are framed correctly as player/packager-side settings, not Gcore settings.
• Verify the new Statistics screenshots match the current Customer Portal UI.
• Writer review and approval