Update Linux desktop integration instructions

**Your Godot version:**
v4.2.1

**Issue description:**
The [Linux desktop integration instructions](https://docs.godotengine.org/en/stable/about/faq.html#linux) could use two changes (IMO):
1. The "Rename and move the Godot binary to a location present in your `PATH` environment variable" section doesn't mention `$HOME/.local/bin/` as an option to place the Godot binary. This location doesn't require administrator privileges, so might be a better option for some users. Certainly worth a mention in any case.
2. Mention how to get the icon to show up. I'm on KDE, and in the main application menu Godot doesn't have an icon (after following the linked instructions). Similar to the "Save this .desktop file to `$HOME/.local/share/applications/`" section, I think explaining where to download and save the Godot icon would be useful too.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/about/faq.html#linux

Start with the Linux desktop integration section of the FAQ at the linked documentation page and review its existing binary and .desktop instructions. Update the section to mention $HOME/.local/bin/ and explain where to obtain and save the Godot icon; the documentation is done when both steps are covered clearly for Linux desktop users.

Written by the indexing model from the issue text.