feat: Add beta and nightly Flatpak sections to download page

[mndscp]

Description

There is an official Flatpak for the development and beta versions of Mixxx, but there is no information about them on the Mixxx download page. I've added a section with instructions for adding the repository and installing the Flatpaks for each.

I've kept wording, style, and notes the same as in the Flatpak section of the stable release.

Testing

I tested all commands in the instructions. They work both when you have one or multiple versions of Mixxx (Flatpak) installed.

Zulip discussion

https://mixxx.zulipchat.com/#narrow/channel/109122-general/topic/Awesome.20Flatpaks

[JoergAtGithub]

Welcome at Mixxx!
As a first-time contributor we need you to sign the Mixxx Contributor Agreement and comment here when you have done so. It gives us permission to distribute your contribution under the GPL v2 or later license and the Apple Mac App Store. It is also helpful for us to have contact information for contributors in case we may need it in the future.

[mndscp]

Hey @JoergAtGithub, I signed the agreement.

[acolombier]

Thank you for looking into this! Still waiting to fix the issue with the repo and the build that are disappearing...

It would be great if we could have the Flatpak section for all release type (beta and stable). I guess we could rename the Flatpak (any Linux distribution) to Flatpak via Flathub (unmaintained) (the plan is eventually ro claim the Flathub listing back once we are more mature with Flatpak management) and add your section as Flatpak (any Linux distribution)

[mndscp]

Hey @acolombier, thanks for the input.

I agree with your suggestion. As both the alpha and beta versions are supposed to be available at all times after the releases will be fixed, I added the beta version Flatpak to the download page as well. Changed the stable version's dropdown label to Flatpak via Flathub (currently unmaintained). And I changed the section headlines for the non-stable versions for more consistency:

Before

- Mixxx 2.5.6 (Stable Version)
- Beta Snapshots (2.6)
- Development Snapshots

After

- Mixxx 2.5.6 (Stable Version)
- Mixxx 2.6 (Beta snapshots)
- Mixxx 2.7 (Development snapshots)

[acolombier]

Just a small link change and this is ready to go, once you have confirmed that the Flatpak repo now works for you :)

Thanks again for improving the situation here!

[acolombier]

LGTM! Thanks for giving this page the TLC it needed!

[daschuer]

LGTM, thank you.