Update the how to build Rebel Documentation locally page

[madmiraal]

To update Rebel Documentation, contributors can make smaller changes here on GitHub directly. However, when making anything more than minor changes, contributors will most likely create a local copy of the Rebel Documentation repository and make changes locally on their computer. When making significant changes to Rebel Documentation (including how to update Rebel Documentation) it is useful to see the effect of your changes before creating a Pull Request. Therefore, before we can make significant updates to Rebel Documentation, one of the first things we need to update is how to build and view the Rebel Documentation locally. This PR creates the Building Rebel Documentation with Sphinx page, which replaces the current Building the manual with Sphinx page.

[DanielaOrtner]

Great work! Thanks for looking at the suggestions so quickly.