Add documentation for troubleshooting flatpak permission issues

[MisterSirCode]

Refer to #2873 (comment) and my issue #20616

This change adds documentation to the troubleshooting FAQ for any future users experiencing cargo file resolving errors with rust-analyzer due to their IDE lacking permission to see the cargo binaries.

[MisterSirCode]

Documentation moved to the vs-code page

Feel free to edit any wording or detail improvements that should be made

[lnicola]

Wait, isn't this the same as the suggestion right above?

If you want to use Flatpak in combination with rustup, the following steps might help:

• using Flatseal, you must add an environment variable called PATH. Set its value to the output from above, appending :~/.cargo/bin, where ~ is the path to your home directory. You must replace ~, as it won’t be expanded otherwise.