feat: Roadmap page

As previously discussed on discord, many people aren't aware of what is being worked on, so having a page with that information wouldn't be a bad idea, especially if it could get more eyes on the Avalonia rewrite.

I see the page being structured like this:

1. Why we don't have a roadmap - mention how the engine is developed and the github roadmap project
2. Current efforts - a list of what the community is currently working on
3. General call for support

For the list of current efforts:

• Avalonia rewrite
• Modernizing outdated parts of the engine
• Updating documentation
• Allowing for code-only development

Q: is this another roadmap that needs to be maintained?
A: the idea is to replace the roadmap page in the docs, so there wouldn't be much maintenance required. Additionally, the content would have to be updated after major changes (like the editor rewrite being completed).

Here's a rough draft I've been working on. Most content is just there as a placeholder.

Start with the repository's build instructions and locate the existing roadmap page in the docs. Use the issue's three-section outline and draft images as the content guide; done means the new page replaces the existing roadmap page and covers the listed current efforts and call for support.