Update beta installation instructions

[confluence]

With the v6 beta release we move away from a separate stream of -beta packages to separate RPM and DEB repositories.

This PR should be backported to both the v5 and v6 release branches.

@kswang1029 @pshnghng0318 I've added you as reviewers to make sure we're all on the same page about the way these repositories will be used. Please note that these instructions are for an SDM / controller installation only, not for desktop users (the general instructions should say something similar, but instead of containers we should probably recommend the AppImage to beta testers).

[daikema]

Overall looks good. Only thing I noticed was capitalization-related, which isn't exactly the most critical of concerns.

[pshnghng0318]

The difference between main page and preview page is sufficiently explained. If people would like to install different versions, using containers seems reasonable to me, but people may want to know more technical details

[confluence]

The difference between main page and preview page is sufficiently explained. If people would like to install different versions, using containers seems reasonable to me, but people may want to know more technical details

@pshnghng0318 for clarification, the step-by-step instructions are intended to cover the common case from start to finish (without going into too much detail about variations). We do mention other installation options in the installation section of the docs. Possibly we should describe a container installation there, but we have previously considered this outside the scope of the instructions. We can see if there is user demand for this information, but I think that typically people install a single version per dedicated VM.