README: link to the documentation is broken

[yasinzaehringer-paradime]

The README reads like this:

## Website

[Visit the Eclipse Theia website](http://www.theia-ide.org) for more information and [the Theia documentation](http://www.theia-ide.org/doc).

However the documentation link here is broken, it should be:

https://theia-ide.org/docs/

[msujew]

I'm surprised that it took 6 years until someone made an issue for that (original commit c231041).

@yasinzaehringer-paradime Are you interested in contributing a fix for that? :)

[eric-sclafani]

Hello, if @yasinzaehringer-paradime is not interested in contributing a fix, may I please be assigned this issue?