[opus] Navbar for documentation is broken in Doxygen 1.8.18

[Ralph Giles]

On Sun, 2020-06-07 at 06:14 +0000, Marcus Asteborg wrote:

> Here is a patch to update documentation to Doxygen 1.8.18 as the
> navbar is broken trying to use the latest Doxygen.

Thanks for the patch! I've merged this in commit fad505e8ed61.

> Also seems we are missing documentation for last releases here:
> 
> https://www.opus-codec.org/docs/

Indeed.

 -r