Improve docs for large file upload limits and timeouts

[nandhan262006]

Summary

This PR improves the "How to increase the upload max file size?" section in the Common Issues page.

What changed

• Expanded documentation on upload size limits beyond upload_max_filesize and post_max_size
• Added guidance for:
  • PHP execution/input time limits
  • Apache/Proxy timeout directives
  • Reverse proxy considerations (e.g., Cloudflare)

Why

Large uploads (~1 GB or more) can fail part-way due to timeout limits even when PHP size limits are configured correctly. These additional details help users troubleshoot upload issues on setups such as XAMPP + Cloudflare Zero Trust.

[Tampa]

Even that doesn't appear to solve the issue. With everything that could possibly impact file uploads set to 2GB I can't seem to get a 150mb file through nginx and php into it.

There are other options for file uploads that involve chunking, like BigUpload library, which I have used before and seems to work well with large GB sized uploads, but that may not work for uploads like ShareX etc. as they may not support chunked uploads.

It is really weird, because normally setting the relevant vars in the webserver and php should be enough to make even really large uploads go through, yet it still returns a 413 that isn't logged anywhere that I could find so far.