PDA general maintenance (NTNet downloader rework) (#79741)

## About The Pull Request

I deleted the documentation file of ModPCs because it was barebones and
had no new information to give that autodoc couldn't. Just to make sure
this isn't a net-negative, I improved on much of the autodoc and
comments in general around ModPC code to help people understand easier
what's going on around it.
I also renamed vars that were too easily confused with other var names,
and reworked the ntnet downloader a little;
- it now has a search bar
- it now has more sections to scroll through, hopefully making it more
accurate and easy to find what you need.
- also organized the apps that were previously shoved in 'other'.
- i also upgraded it to a .tsx because why not

video demonstration


https://github.com/tgstation/tgstation/assets/53777086/cbba4c1c-b8a8-4ba4-8628-aea8389999fc

## Why It's Good For The Game

Adds in a lot of comments that were previously missing, clears up some
sources of confusion within ModPC code, and improves NTNet Downloader,
something I've procrastinated on doing for a very long time now.

## Changelog

🆑
qol: NTNet Downloader now has a search bar, and programs are now better
sorted.
/🆑

code/modules/modular_computers/file_system/programs/messenger/messenger_program.dm

@@ -8,8 +8,8 @@
 /datum/computer_file/program/messenger
 	filename = "nt_messenger"
 	filedesc = "Direct Messenger"
-	category = PROGRAM_CATEGORY_MISC
-	program_icon_state = "command"
+	downloader_category = PROGRAM_CATEGORY_DEVICE
+	program_open_overlay = "command"
 	extended_desc = "This program allows old-school communication with other modular devices."
 	size = 0
 	undeletable = TRUE // It comes by default in tablets, can't be downloaded, takes no space and should obviously not be able to be deleted.