I suggest to

• add a make.bat file to ./doc/ to make is easier for windows users to make the documentation
• move the existing Makefile from ./doc/src to ./doc/ to access the file faster and store the make files at the same place
• rename the folder for the created html from _build/ to build. By this change the created files are covered by the already existing .gitignore which prevents adding generated documentation to the repository by accident.

If renaming _build is not wanted, a .gitignore file could be added to ./doc/ to achieve the same result.