Unit test documentation is outdated

[CookiePLMonster]

Describe the enhancement you're suggesting.

A few things that I could spot while figuring out how to set up the unit tests on my Flipper with the help of the docs:

1. The instructions mention test_index.c twice, but that file doesn't exist. What is the current replacement for it?

2. The instructions state

   Flash the firmware using your preferred method, including SD card resources

   but FBT's [Debug:unit_tests] Flash (USB) task does "./fbt FIRMWARE_APP_SET=unit_tests FORCE=1 flash_usb" and not flash_usb_full, thus skipping resources.

3. The docs mention updater_package, but FBT's vscode task skips it. Which of the two is correct?

Anything else?

No response