docs: update for NerdFonts v3

[cstrahan]

This fixes the docs, along the lines of: https://github.com/goolord/alpha-nvim/pull/209

[cstrahan]

Prior to this change:

~/src/nvim_plugins/alpha-nvim
% nerdfix check --recursive . 2>/dev/null
note: Found obsolete icon U+F659
    ┌─ ./doc/alpha.txt:277:38
    │
277 │              dashboard.button( "q", "  Quit NVIM" , ":qa<CR>"),
    │                                      ^ Icon 'mdi-close_circle_outline' is marked as obsolete
    │
    = You could replace it with:
          1. 󰅚 U+F015A md-close_circle_outline
          2. 󰢃 U+F0883 md-close_circle_multiple_outline
          3. 󰏦 U+F03E6 md-pause_circle_outline
          4. 󰮕 U+F0B95 md-eye_circle_outline

note: Found obsolete icon U+F659
    ┌─ ./doc/alpha.txt:322:40
    │
322 │                 startify.button( "q", "  Quit NVIM" , ":qa<CR>"),
    │                                        ^ Icon 'mdi-close_circle_outline' is marked as obsolete
    │
    = You could replace it with:
          1. 󰅚 U+F015A md-close_circle_outline
          2. 󰢃 U+F0883 md-close_circle_multiple_outline
          3. 󰏦 U+F03E6 md-pause_circle_outline
          4. 󰮕 U+F0B95 md-eye_circle_outline