docs, remove 'guifontset' #11708

- remove redundant autocmd list
  This "grouped" list is useless, it only gets in the way when searching
  for event names.
- intro.txt: cleanup
- starting.txt: update, revisit
- doc: `:help bisect`
- mbyte.txt: update aliases 1656367b90. closes #11960
- options: remove 'guifontset'. Why:
  - It is complicated and is used by almost no one.
  - It is unlikely to be implemented by Nvim GUIs (complicated to parse,
    specific to Xorg...).

src/nvim/options.lua

@@ -1021,15 +1021,6 @@ return {
       redraw={'ui_option'},
       defaults={if_true={vi=""}}
     },
-    {
-      full_name='guifontset', abbreviation='gfs',
-      type='string', list='onecomma', scope={'global'},
-      deny_duplicates=true,
-      vi_def=true,
-      varname='p_guifontset',
-      redraw={'ui_option'},
-      defaults={if_true={vi=""}}
-    },
     {
       full_name='guifontwide', abbreviation='gfw',
       type='string', list='onecomma', scope={'global'},