Readme: implement alternate way to build EPUB

Workaround for EPUB build Sphinx build:

- no longer use "epub" branch
- remove "hidden" TOC directive on "build" branch via sed, build on branch, then
  delete temporary branch

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