Linux: Display a warning if Obsidian is installed using Flatpak

[Taitava]

Flatpak isolates Obsidian from the rest of the operating system, preventing external programs from being executed. It's an understandable security choice, but it also prevents intentional access to external programs.

https://github.com/Taitava/obsidian-shellcommands/discussions/367#discussioncomment-6844635

I also asked for help in Discord so that the plugin could detect if Obsidian was installed using Flatpak. The plugin could then show a tiny notice in the settings modal, warning users that the shell commands might not work as expected when Flatpak is used. Will see if such a feature could be developed.

TODO:

• [x] Add an informational warning to the settings modal.
• [x] Test on Linux: Install Obsidian with Flatpak and check that the warning appears.
• [x] Add to changelog.
• [x] Create a documentation page about Flatpak.
• [ ] Try to create a guide that tells how Flatpak can be configured to whitelist some external programs.
• [x] Also point out that sometimes it's just easier to install Obsidian by using other means.

[Taitava]

Done, but I did not create this guide:

Try to create a guide that tells how Flatpak can be configured to whitelist some external programs.

Will see if I get to do that later.

[Taitava]

Released.