Add a troubleshooting guide (docs/TROUBLESHOOTING.md)

[alimaandev]

Problem

Common setup/render failures (missing ffmpeg, missing Node.js, Remotion render errors, OAuth expiring, Ollama not running) currently surface as bare exceptions with little guidance.

Task

Write docs/TROUBLESHOOTING.md covering the most frequent issues, with symptoms → cause → fix, and link it from README.md (FAQ and/or Contributing) and from CONTRIBUTING.md.

Suggested sections

• ffmpeg not found / pydub warning
• Node.js not installed or remotion/node_modules missing
• AssemblyError/Remotion render failures (incl. the 'no usable output file' sanity check)
• YouTube OAuth expired / client_secrets.json missing
• Ollama connection refused
• LLM 'empty response' or repeated JSON parse failures

Acceptance criteria

• One file, clearly linked from README
• Every listed fix is verifiable (command or settings path)
• No secrets or placeholder API keys in examples

Files

• docs/TROUBLESHOOTING.md (new)
• README.md
• CONTRIBUTING.md