fix(runtime)!: move "tohtml" to pack/dist/opt/ #34557

Problem:
The "tohtml" plugin is loaded by default.

Solution:
- Move it to `pack/dist/opt/nvim.tohtml/`, it is an "opt-in" plugin now.
- Document guidelines.
- Also revert the `plugin/` locations of `spellfile.lua` and `net.lua`.
  That idea was not worth the trouble, it will be too much re-education
  for too little gain.

test/functional/plugin/tohtml_spec.lua

@@ -186,6 +186,7 @@ describe(':TOhtml', function()
   local screen
   before_each(function()
     clear({ args = { '--clean' } })
+    n.command('packadd nvim.tohtml')
     screen = Screen.new(80, 80, { term_name = 'xterm' })
     exec('colorscheme default')
   end)