Docs: remove beta warning from config file's `build` key
I think we can stop calling it beta since it has been working pretty well for
some months already and we are happy with it.
@@ -302,12 +302,6 @@ The path to the Conda environment file, relative to the root of the project.
build
~~~~~
-.. warning::
-
- This functionality is in beta. If you find any inconsistencies
- or have feedback, please `open an
- issue <https://github.com/readthedocs/readthedocs.org/issues/>`_.
-
Configuration for the documentation build process.
This allows you to specify the base Read the Docs image
used to build the documentation,
