docs: Add fish_title and fish_greeting

doc_src/cmds/fish_greeting.rst

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+.. _cmd-fish_greeting:
+
+fish_greeting - display a welcome message in interactive shells
+===============================================================
+
+Synopsis
+--------
+
+::
+
+  function fish_greeting
+      ...
+  end
+
+
+Description
+-----------
+
+When an interactive fish starts, it executes fish_greeting and displays its output.
+
+The default fish_greeting is a function that prints a variable of the same name (``$fish_greeting``), so you can also just change that if you just want to change the text.
+
+While you could also just put ``echo`` calls into config.fish, fish_greeting takes care of only being used in interactive shells, so it won't be used e.g. with ``scp`` (which executes a shell), which prevents some errors.
+
+Example
+-------
+
+A simple greeting:
+
+::
+
+  function fish_greeting
+      echo Hello friend!
+      echo The time is (set_color yellow; date +%T; set_color normal) and this machine is called $hostname
+  end

doc_src/cmds/fish_title.rst

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+.. _cmd-fish_title:
+
+fish_title - define the terminal's title
+========================================
+
+Synopsis
+--------
+
+::
+
+  function fish_title
+      ...
+  end
+
+
+Description
+-----------
+
+The ``fish_title`` function is executed before and after a new command is executed or put into the foreground and the output is used as a titlebar message.
+
+The first argument to fish_title contains the most recently executed foreground command as a string, if any.
+
+This requires that your terminal supports programmable titles and the feature is turned on.
+
+
+Example
+-------
+
+A simple title:
+
+
+
+::
+
+   function fish_title
+       set -q argv[1]; or set argv fish
+       # Looks like ~/d/fish: git log
+       # or /e/apt: fish
+       echo (fish_prompt_pwd_dir_length=1 prompt_pwd): $argv; 
+   end
+