First structure for the website

[theogf]

The idea of this PR is to start constructing a small structure for the website that can be later filled PR by PR.
Here is the proposal:

• Have a welcome page (index.md) to redirect user to all the content of the website
• Have a page for each package (AbstractGPs.jl, KernelFunctions.jl) which would contain some ressources such as Theory, small examples etc (the range is quite free and adaptable). For example this would be the location for the PR Visual example for kernel via AbstractGPs KernelFunctions.jl#133
• Have a tutorial page redirecting to a (soon to be) large collection of tutorials generated by Literate.jl.
• I removed PkgPage.jl to switch back to Franklin.jl. I realised quickly that PkgPage.jl is in fact made for having one page which is not fitting our current needs. I kept the current theme but readapted some things to make multiple pages work

[EDIT] Additionally, I improved the workflow for changes:

• Now every time a new PR is made a different website is deployed via netlify and a github action will comment with the new link to visit to check out changes, this does not affect in any way the current website hosted on github.

This PR aims also at smoothing some hard edges like : changing the icons, making sure all links are correct etc...

[github-actions]

Once the build has completed, you can preview your PR at this URL: https://juliagps.netlify.app/previews/PR2/