[thunderbird] Add download link to downloads page, and update documentation

[martgil]

For documentation:

I noticed that the Flatpak version of Thunderbird does not work pretty well when installing debug addons and possibly xpi version of extensions (due to permissions, perhaps -- not sure why as it's quite difficult to dive in). So for a solution for Linux users, to avoid any issues, use the snap version of Thunderbird using "snap install thunderbird".

One may also want to double check if its a Flatpak version is by checking "flatpak list" and uninstall the thunderbird version of it using "flatpak uninstall Thunderbird" (take note of the capitalized app name as it also serves its app id)

[martgil]

Inform the following user about the availability of the extension in Thunderbird: https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgzQVxbrTfRQZhQJLjltQzMwGlHQg