Use mdBook for the application documentation

[josecelano]

For now, we have the application documentation on some markdown files in the docs folder:

https://github.com/torrust/torrust-index-frontend/blob/develop/docs/index.md

But we are not publishing it anywhere.

For the tracker and backend, we have the docs on https://docs.rs/.

Since we do not have any Rust crate for this repo, we could use a mdBook:

https://github.com/rust-lang/mdBook

and publish it to Github Pages.

The documentation for this repo could be extensible if we include here:

• User guide
• Deployment guides (maybe this one should go on the Index project)

cc @da2ce7

[da2ce7]

I guess the only counter-proposal for this one is to merge the #73 and publish a crate.

[josecelano]

Alternatives:

• https://vitepress.dev/