Update HTTPS Nginx Documentation with tips - upload size and custom splash

[matthewhand]

This PR refactors the https-nginx.md documentation to streamline usage instructions and improve usability, aligning with the project's contribution guidelines.

Changes:

1. Integrated Usage Instructions:

   • Consolidated reload instructions under a new "Applying Changes" section in the main documentation.
   • Improved clarity by embedding reload commands where configuration changes are discussed.

2. Streamlined Splash Screen Customization:

   • Added tabs for local and remote splash screen configurations in the "Customize Splash Screen and Favicon" section.
   • Provided clear steps for both hosting methods, ensuring ease of implementation.

3. Additional Tips:

   • Added a standalone tip for increasing upload size using client_max_body_size, addressing common user needs.

Why This Change?

• Aligns with Atomic PR best practices by focusing on a single objective: improving the clarity and usability of the HTTPS Nginx documentation.
• Enhances user experience by consolidating related content into a single cohesive document.
• Maintains logical flow and simplicity, reducing cognitive load for readers.

Related Discussions/Issues:

• N/A (No prior discussion was opened for this minor refactor; please advise if one should be started retroactively.)

Checklist:

• Updates adhere to the existing project style and conventions.
• Documentation reflects changes and includes clear, actionable steps.
• No external dependencies were added.

[tjbck]

Let's leave a note here for sponsorship here.

[matthewhand]

something like this?...

### Important Note on Sponsorship and Attribution

While Nginx can be used to rebrand the Open WebUI (e.g., replacing the splash screen or favicon), it’s important to acknowledge that this open-source project relies on the generous support of its sponsors. Rebranding without attribution can undermine the efforts of those funding the project’s ongoing development.

**Recommendation:**  
If rebranding is implemented using the methods described here, please provide some attribution back to the Open WebUI project (e.g., by mentioning the project in your modified version). This ensures fairness and continued support for the open-source community.