Add the vcs prompts to sphinx docs

sphinx_doc_src/cmds/fish_vcs_prompt.rst

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+fish_vcs_prompt - output vcs information for use in a prompt
+============================================================
+
+Description
+-----------
+
+The fish_vcs_prompt function can be used to display information about the current vcs repository, if any.
+
+It calls out to vcs-specific functions. The currently supported ones are:
+
+- fish_git_prompt
+- fish_hg_prompt
+- fish_svn_prompt
+
+If a vcs isn't installed, the respective function does nothing.
+
+For more information, see their documentation.
+
+\subsection fish_vcs_prompt-example Example
+
+A simple prompt that displays vcs info::
+
+    function fish_prompt
+        ...
+        set -g __fish_git_prompt_showupstream auto
+        printf '%s %s$' $PWD (fish_vcs_prompt)
+    end