[Draft] Docs setup

[mbasaglia]

Setting up a build system for documentation with schema integration.

Output: https://lottiefiles.github.io/lottie-spec/specs/composition/

[mbasaglia]

For now it uses the default style of one of the mkdocs themes, that will need to be updated before merging

[mbasaglia]

As part of this I'm expanding some descriptions on the schema and renaming some objects to increase clarity.

[mbasaglia]

This is an example of what a generated page would look like. The description and property table are pulled from the schema, and an interactive example shows what the properties do.

[fmalita]

Thanks Mattia, this looks great!

[Aidosmf]

thanks

[jbeta51]

LGTM, harder to review since this is a large migration, but checking the site everything looks good.