Add design document for web functionality

[TimJentzsch]

In a GitHub discussion, create a design doc for all web features of the CLI.
They should capture the vision of this sub-project and invite for a broader discussion with the Bevy community.

It should contain:

• Motivation: Why is this feature needed, which problems does it solve?
• Summary: Summary of what we are trying to build
• Design goals: Overarching strategy used to develop the feature
• MVP features: What we need for an initial release
• Future features: Outlook of what could be built on top of the basics

[BD103]

In a GitHub discussion, create a design doc for all web features of the CLI.

Do you want me to enable Github Discussions for this repository, or do you want to just put it in Bevy's? The latter will probably get more visibility.

[TimJentzsch]

Honestly a good question, I'm not sure which one would be better...

[TimJentzsch]

I created a HackMD to draft the design document, feel free to review and improve it!

CC @BD103, @DaAlbrecht, @alice-i-cecile