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.