docs: add tutorial for customizing themes (opening early for reference)

[silentoplayz]

Description

This PR adds a new documentation page that provides a comprehensive guide on how to customize and add new themes to Open WebUI, particularly animated themes powered by Particle.js.

The new page is located at docs/tutorials/theming.mdx and covers how users can propose new themes and provides a step-by-step guide for developers to create and register their own themes, specifically animated themes powered by Particle.js.

Motivation

The goal of this new documentation page is to empower the community to contribute to the look and feel of Open WebUI by providing clear and actionable instructions on theme creation. This will hopefully lead to a richer and more diverse set of themes available for all users.

How to Test

Check out this PR.

Install the dependencies:

npm install

• Build the documentation site to ensure there are no errors:

npm run build

• Serve the built site locally:

npm run serve

• Open your browser and navigate to the locally served documentation.
• Find the new "Customizing Themes" page under the "Tutorials" section in the sidebar.
• Verify that the content renders correctly and is easy to follow.

My apologies for opening up both this PR and #675 on the main branch. I need to update my local dev environment so that my changes are being made on the dev branch.

This PR links to a PR I've yet to submit over on the main repo!