docs: add troubleshooting guide

[Suraj-H675]

What problem does this solve?

HTTPie has 38k stars and millions of users but no troubleshooting guide. Users hit SSL errors, proxy issues, connection timeouts, and JSON parse errors regularly — there's no central place that explains what went wrong and how to fix it.

What was changed and why?

Added docs/troubleshooting.md with six sections:

• Collecting Debug Information — how to use --debug for useful bug reports
• Exit Status Codes — all HTTPie exit codes (0, 1, 2, 3, 4, 5, 6, 7, 130) with meanings
• Connection Problems — connection refused/timeout, SSL errors (cert verify failed, version mismatch), proxy errors (auth failures, tunnel errors)
• Request Errors — JSON parse errors with shell quoting fixes, invalid HTTP status codes, Content-Encoding gzip download errors (refs Fix incomplete download error when Content-Encoding gzip is present #1843, Fix false 'Incomplete download' error when Content-Encoding is set #1707)
• Authentication Errors — missing env vars, SSL client cert auth
• Shell and Platform Issues — httpie vs http on macOS, Windows path expansion, Python version requirements

Exit status codes sourced from httpie/status.py, SSL options from httpie/ssl_.py, error messages from actual code paths in urllib3/requests stack.

How was this tested?

• Diff verified: only docs/troubleshooting.md added, +257 lines
• Exit status codes sourced from ExitStatus enum in httpie/status.py
• SSL version options verified against httpie/ssl_.py's SSL_VERSION_ARG_MAPPING
• Markdown formatting follows existing httpie docs style

Checklist

• I have read the CONTRIBUTING.md guidelines
• My changes match the project's documentation style
• All existing tests pass locally (docs-only change)
• Referenced issue numbers for known bugs (Fix incomplete download error when Content-Encoding gzip is present #1843, Fix false 'Incomplete download' error when Content-Encoding is set #1707)

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This contribution was created with AI assistance following structured guidelines, with human review and approval at each step.