Improve animation documentation

[mwestphal]

Context
F3D doc has a short paragraph about animation: https://f3d.app/doc/user/USAGE.html#animations
While informative, it doesn't convey the full complexity of interacting with animation in F3D and animations in 3D formats in general.

Let's expand this doc in its own page

Suggested solution

• Add a dedicated markdown file to cover how are animations defined in 3D models and how to interact with them in F3D
• Move the short paragraph there as well
• Add links wherever needed

Skills

• Markdown

[mwestphal]

discussed here initially: #1242

[mwestphal]

@kathleenhang wanna take this one ?

[kathleenhang]

@kathleenhang wanna take this one ?

Hi Mathieu, sure thing! Let me know if there are any best practices I should know about for writing F3D documentation. Otherwise, I will go ahead and add my notes as I see best fit !

[mwestphal]

For now animation is just a single paragraph.

I we want to expand this doc, we may be better adding a dedicated .md file for it, which means it should be listed in the index.