Add the ability to view a book for older versions of the library.

[ThsUsr]

At the moment, as I understand it, it is impossible to view the book for version 0.6 after the release of 0.7. This is a problem, because after the release of a new version of the library, the ability to view books for older versions of the library is lost, and they cannot be viewed on the site.

A good example of what I'm talking about is the yew/dioxus documentation.

[gbj]

You can access the 0.6 version by cloning this repo, checking out dacf468, and running mdbook serve locally.

If you think it's important to have multiple versions hosted publicly online, please consider looking into how we can do that easily and making a suggestion here! This is a community project so I have pretty limited capacity to work on things like this, but they tend to be very good ways for people to get involved.

[bicarlsen]

We could indicate the leptos version the book corresponds to, with a list of the commits to previous versions.

[bicarlsen]

For reference, it appears Yew uses Docusaurus and Dioxis uses itself.

For mdbook it seems you just build each commit of interest and host the artifacts. In the linked Issue an example of this is given.