runtime(doc): add termdebug tag, remove term "floating window"

runtime(doc): add termdebug tag, remove term "floating window"

Problem:
- When I type `:h termdebug`, I will expect to see the introduction of
  the termdebug plugin. But instead, it shows me document of
  `termdebug_wide`, and I have to scroll up quite much to find the
  introduction.
- `:h popup` says `floating-window`? Why? As I have tried both features
  (of Vim and Neovim), I think they are _very different_ things, even
  more different than job features in Vim and Neovim.

Solution:
- In `:h terminal.txt`, add tag `*termdebug*` to the introduction of
  termdebug plugin.
- In `:h popup.txt`, "floating window" -> "popup window".

closes: #19135

Signed-off-by: Phạm Bình An <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>