Add beta flatpak link and installation guide

Signed-off-by: Matthew J. Milner <[email protected]>

source/install/flatpak.md

@@ -0,0 +1,80 @@
+(install-flatpak)=
+
+# Flatpak
+
+[Flatpak](https://flatpak.org/) is an app packaging solution for Linux that solves the issue of differences between distributions and allows a single app to run on all Linux desktops.
+This means not just different distros but also both older and newer distros, so if you were stuck using an old version of Avogadro until now, give the flatpak a try.
+
+The Avogadro team is aiming for flatpak to become the primary method of distribution of Avogadro 2 on Linux.
+(Though distribution via other routes will of course continue.)
+
+As well as the usual benefits of flatpaks such as stability and the possibility for sandboxing, by using the Avogadro flatpak you can easily keep it up-to-date with the latest versions via updates.
+
+If you've never used flatpak before, the first step is to make sure you have the `flatpak` package, but you likely already do -- it is installed by default on most modern distros.
+
+Avogadro is distributed through [Flathub](https://flathub.org/apps/org.openchemistry.Avogadro2) on two branches: a [`stable`](install-flatpak-stable) branch, which contains the most recent release, and a [`beta`](install-flatpak-beta) branch, which is similar to the "nightly" versions on other platforms and has the latest features and bug fixes.
+
+
+(install-flatpak-stable)=
+
+## Stable
+
+In many cases, flatpak is set up with Flathub out-of-the-box.
+If not, and you haven't set it up yourself manually before, you can follow the instructions [here](https://flathub.org/setup).
+
+Once flatpak is set up, you can easily install Avogadro from Flathub in one of two ways:
+
+1. Through your distro's "app store" (e.g. KDE Discover or GNOME Software) -- just search for Avogadro, or click on install on the [Flathub page](https://flathub.org/apps/org.openchemistry.Avogadro2)
+2. On the command line using:
+```shell
+flatpak install org.openchemistry.Avogadro2
+```
+
+After installation, Avogadro should show up in your applications menu/drawer like any other app.
+If not, you can always run a flatpak using:
+```shell
+flatpak run org.openchemistry.Avogadro2
+```
+
+:::{tip}
+By default, `flatpak` works on the system installation (see the [flatpak usage guide](https://docs.flatpak.org/en/latest/using-flatpak.html)), just like `apt`, `dnf` and friends.
+This means that to add repos and install Avogadro using the commands above, or using your distro's software manager, you may need `sudo` privileges.
+
+On the whole it is recommended to do things this way, but it might not always be possible.
+To do things just for your user without `sudo` privileges, use the `flatpak` command with the `--user` flag, e.g.
+```shell
+flatpak --user install org.openchemistry.Avogadro2
+flatpak --user run org.openchemistry.Avogadro2
+```
+:::
+
+
+(install-flatpak-beta)=
+
+## Beta
+
+Though our nightly builds are generally pretty robust, it is not allowed to distribute nightly, potentially unstable builds over Flathub, so instead we are providing an "almost-nightly" `beta` branch to fulfil the same purpose.
+It will receive the latest updates at regular intervals.
+
+:::{warning}
+While it is possible, it is strongly recommended that you do not have both the `stable` and `beta` version of Avogadro installed at the same time, to avoid complications.
+:::
+
+To get the `beta` branch, you will first have to add the `flathub-beta` repository, which is located at `https://flathub.org/beta-repo/flathub-beta.flatpakrepo`.
+
+On the command line this is done using:
+```
+flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
+```
+
+If using a GUI software manager to manage your flatpaks, they often also provide a way to add a repository.
+
+Once the repo has been added, you can install the Avogadro `beta` flatpak in the usual way, done on the command line using:
+```shell
+flatpak install org.openchemistry.Avogadro2//beta
+```
+
+The flatpak can then be run in the usual way.
+
+```{toctree}
+```

source/install/index.md

@@ -151,7 +151,7 @@ Download AppImage
 :::{button-link} https://flathub.org/apps/org.openchemistry.Avogadro2
 :color: primary
 :outline:
-Download Flatpak
+Install the Flatpak
 :::
 
 :::{button-link} https://repology.org/project/avogadro2/versions
@@ -172,6 +172,12 @@ Check your distro's repositories
 :outline:
 Nightly AppImage
 :::
+
+:::{button-ref} install-flatpak-beta
+:color: secondary
+:outline:
+Install the Beta Flatpak
+:::
 ::::
 
 
@@ -255,5 +261,6 @@ hidden: true
 caption: Installation
 ---
 self
+flatpak
 versions/index
 ```