EPUB: implement Sphinx bug workaround in Makefile

- stash unstaged changes if any
- remove "hidden" TOC directive via sed
- reset master branch to initial state after build
- restore stashed changes if any (test grep result as "git stash list" does not
  seem to return any exit code)
- remove steps for manual workaround from README

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>