This now exists as a GitBook too: https://b3m2a1.gitbooks.io/mathematica-tutorial/content/

I wrote a little script to turn a website generated with the SiteBuilder package into a GitBook ready website:

gitBookCopyContent[srcDir_, bookDir_] :=
  With[{
    main = FileNameJoin@{srcDir, "content"}
    },
   Export[
    FileNameJoin@{bookDir, "README.md"},
    StringSplit[
      Import[FileNameJoin@{main, "pages", "About.md"}, "Text"],
      "\n\n",
      2
      ][[2]],
    "Text",
    CharacterEncoding -> "UTF8"
    ];
   Map[
    If[! MatchQ[FileNameTake[#], "pages" | "posts" | ".git"],
      With[{f = FileNameJoin@{bookDir, StringTrim[#, main]}},
       Which[DirectoryQ@#,
        If[FileExistsQ@f,
         DeleteDirectory[f, DeleteContents -> True]
         ];
        CopyDirectory[#, f],
        MatchQ[FileExtension[#], "png" | "jpeg"],
        CopyFile[#, f, OverwriteTarget -> True]
        ]
       ]
      ] &,
    FileNames["*", main]
    ]
   ];
gitBookExportMD[outdir_, bookDir_, content_] :=
  KeyValueMap[
   Function[
    With[{f =
       FileNameJoin@Flatten@
         {
          bookDir,
          FileNameSplit@
           StringTrim[#, outdir]
          }
      },
     If[! DirectoryQ@DirectoryName@f,
      CreateDirectory[DirectoryName@f, 
       CreateIntermediateDirectories -> True]
      ];
     Export[
      f,
      StringJoin[
       StringReplace[#2,
         { 
          "{{" -> "{ {",
           "}}" -> "} }",
          "{filename}" ->
           URLBuild@
            ConstantArray["..", 
             FileNameDepth[StringTrim[#, outdir]] - 2
             ]
          }
         ] /.
        StringExpression[a___] :> a
       ],
      "Text",
      CharacterEncoding -> "UTF8"
      ]
     ]
    ],
   content
   ];
gitBookMakeSummary[outdir_,  bookDir_, metas_] :=

  Module[{data,  cells, nb},
   data =
    GroupBy[First -> Last] /@

     GroupBy[#[[1, 1]] & -> (#[[1, 2]] -> #[[2]] &)]@
      KeyValueMap[
       URLParse[#2["Path"], "Path"][[;; 2]] ->
         <|

          "Path" ->
           URLBuild[FileNameSplit@StringTrim[#, outdir]],
          "Title" -> #2["Title"]
          |> &,
       metas
       ];
   cells =
    KeyValueMap[
     Cell[
       CellGroupData[Flatten@{
          Cell[#, "Section"],
          KeyValueMap[
           Cell[
             CellGroupData[
              Flatten@{
                Cell[#, "Subsection"],
                Map[
                 Cell[
                   TextData[
                    ButtonBox[#Title,
                     BaseStyle -> "Hyperlink",
                    ButtonData ->
                    {

                    FrontEnd`FileName[
                    Evaluate@URLParse[#Path, "Path"]], 
                    None
                    }
                    ]
                    ],
                   "Item"
                   ] &,
                 #2
                  ]
                }
              ]
             ] &,
           #2
           ]
          }
        ]
       ] &,
     data
     ];
   CheckAbort[
    nb =
     CreateDocument[
      Prepend[
       cells,
       Cell[
        TextData[
         ButtonBox[
          "Introduction",
           BaseStyle -> "Hyperlink",
          ButtonData ->
           {
            "README.md", 
            None
            }
          ]
         ],
        "Item"
        ]
       ],
      Visible -> False,
      StyleDefinitions -> 
       FrontEnd`FileName[{"BTools"}, "NotebookToMarkdown.nb"],
      NotebookFileName ->
       FileNameJoin@{bookDir, "SUMMARY.nb"}
      ];
    NotebookMarkdownSave[nb];
    NotebookClose[nb];
    DeleteFile[FileNameJoin@{bookDir, "SUMMARY.nb"}];,
    NotebookClose[nb]
    ];
   ];
gitBookBuild[srcDir_, bookDir_] :=
  Module[
   {outdir, content},
   gitBookCopyContent[srcDir, bookDir];
   outdir =
    FileNameJoin@{srcDir, "content", "posts"};
   content =
    AssociationMap[
     With[{split = StringSplit[Import[#, "Text"], "\n\n", 2]},
       <|
        "Meta" ->
         Association@
          Map[
           Rule @@ StringTrim@StringSplit[#, ":", 2] &,
           StringSplit[split[[1]], "\n"]
           ],
        "Body" ->
         split[[2]]
        |>
       ] &,
     FileNames["*.md", outdir, \[Infinity]]
     ];
   gitBookExportMD[outdir, bookDir, content[[All, "Body"]] ];
   gitBookMakeSummary[outdir, bookDir, 
    SortBy[
     content[[All, "Meta"]],
     ToExpression[StringSplit[#ID, "."]] &
     ]
    ];
   bookDir
   ];

This doesn't mirror over the theme, but it gets the job done

I use it like this:

gitBookBuild[
  FileNameJoin@{$WebSiteDirectory, "tutorial"},
  FileNameJoin@{$UserDocumentsDirectory, "GitHub", 
    "mathematica-tutorial"}
  ];

and then commit the mirrored repo on GitHub that GitBook builds from