[dev-v5] docs: add a link to GitHub repo

[AClerbois]

Hi guys,

I suggest to add a link to the GitHub repository.

This should be more easily and get a way to quickly come from doc to repo.

I imagine to way to make it :

In the footer :

In the header (as displayed in the current doc website):

About the link, I propose to target the current v5's branch : https://github.com/microsoft/fluentui-blazor/tree/dev-v5

[dvoituron]

Yes. We are waiting you :-)

[AClerbois]

@dvoituron which one do you prefer ? Header / Footer / both ?

[dvoituron]

Header / right, like in v4

[AClerbois]

I'm gonna to do it 👍