docs(tui): rename term.txt, nvim_terminal_emulator.txt #30398

Problem:
It has long been a convention that references to the builtin terminal UI
should mention "tui", not "term", in order to avoid ambiguity vs the
builtin `:terminal` feature. The final step was to rename term.txt;
let's that step.

Solution:
- rename term.txt => tui.txt
- rename nvim_terminal_emulator.txt => terminal.txt
- `gen_help_html.lua`: generate redirects for renamed pages.

runtime/doc/terminal.txt

@@ -1,4 +1,4 @@
-*terminal_emulator.txt*   Nvim
+*terminal.txt*   Nvim
 
 
 		 NVIM REFERENCE MANUAL    by Thiago de Arruda

runtime/doc/tui.txt

@@ -1,4 +1,4 @@
-*term.txt*      Nvim
+*tui.txt*      Nvim
 
 
                             NVIM REFERENCE MANUAL